For that, you want/need an FF body and lens. Yes, crop lenses are occasionally shorter BUT once you're at a 180° field of view (an 8mm fisheye on a full frame/35mm film body), you're pretty much at the limit.
You can get 180Deg AoV on crop... just.... it takes a Sigma 4.5mm Fish, which I believe is still the only 180 full-round for APS-C, and a chunk load of dosh for likely use one will ever have, but still... at around the £500 mark, it still came in a less than going Full-Frame, to get 180 Deg AoV and Full round pictures; even though you can pick up 'budget' fish for full frame for under £200.. all-be it manual focus... but then WHY they made the Siggy 4.5 Auto-Focus is still something of an mystery to me! With a closest focus distance of oooh... next to nothing, and hyperfocal before arms length, even at f2.8 I would have to try really REALLY hard to get anything oof-ed! Vasaline perhaps? But you cant put a filter on the front.... maybe stuffing the filter draw with bubble-wrap? I dunno!
Other Siggy in the bag is the 8-16, 'rectilinear', which again claims, at least when I bought mine, to be the widest UWA for APS-C with a 115Deg AoV, equivalent to approx 12mm on full-frame, and and tending into the murky depths of fishing, even there. (The Fish eye for my film cameras, that prompted this perversion to chase the wide side in the first place! is 12mm, and delivers the cropped circle image)
So you can go as wide on APS-C as on Full-Frame, and there isn't a lot of price incentive to go one way or t'other really.... maybe if you are looking very hard to justify the switch to FF, then it offers some excuse, and 'maybe' some slightly wider choice of lenses, but, its hard to say that you 'must' go full-frame to shoot that wide; you don't .
ANYWAY Back on topic; OP has now admitted that their desire for a second lens, is pretty much just because they have an interchangeable lens mount, and want to be able to use it!
In days of yore, cameras seldom had 'easily' interchangeable lenses; they had a lens, and that was it! IF you swapped camera... you by default got a new lens to go with it, and it wasn't 'such' a long time ago, with the popularity of zoom lenses, that folk suggested that having just a 2x zoom, say 35-70, that covered 'standard' angle, and some mild wide and mild tele, which oh-so often never came off the camera, rather made the interchangeable lens mount of system cameras rather redundant.... in fact, it was something Olympus recognised as early as the late 1980's, with I think it was the IS range of 'all in one' super-zoom SLR's that pioneered modern 'Bridge' cameras....
As for future-proofing? Good luck on that front... I don't think you ever really can, the future, is by nature, uncertain! And does it really matter? You buy a camera to take pictures; if it does, then job jobbed. If they make a new camera in a years time... well, has the one you got stopped being able to take pictures? Should STILL do the job.
If in a years time, you decide that the 'new' camera is an absolute must have for some reason; then you will go buy it... whether that means swapping all the lenses and accessories you have acquired to go with it, and buying over to suit this new must have super-widget, then so be it.... if not, you wont.... and hoping or trying to find some 'economy' in the middle scavenging some utility from what you already have, is just that, meddling in the middle scavenging, and 'hope'... and making the matter more complicated and difficult than needs, just for the sake of.
As said, once upon a time, you bought a camera, it had a lens, and that was it! End of.. no choice, no argument and no pontificating or indecision! Pays your money and take your chances, but live in the here and now, not some elusive future that may never happen!
Particularly at wide to normal focal lengths, a ‘wide’ FX lens would simply be a ‘normal’ length lens on crop, and a ‘standard ’ lens becomes a bit too long to be standard, not long enough to be usefully long.
By all means buy FX telephoto lenses. But expect to be disappointed as they ‘shrink’ when you put them on a FF body.
@Teflon-Mike , I guess you are right in the regards to future proofing, and living in the here and now with what works is the main thing. Like you say, my camera wouldn't stop taking photos with what it uses just because something new has come out.
It is partly because I can change lenses I want to be able to do so- but this was the reason I bought a DSLR. I did mention the surrounding areas I would like to be able to photograph from my hometown, as it has a great view all around of various things. I would like to be able to capture these, and not end up with a pixelated mess when cropped. The wind farm I can see would be one great thing I would like to be able to capture sometime.
If getting a zoom lens in the range I mentioned is a waste of money, is there some kind of converter I can fit that would enable my current lens to be used for when I do need that extra reach?
I've not forgotten your suggestion of getting a lot of used, older glass for a fraction of the money to experiment with, this is another option I may well choose, or getting the used 55-200MM Nikon lens. Just was wondering if there was a third option that would work. When I am into something, I enjoy learning as much as I possibly can about what can be used, and how these things work- I think this may stem from my other hobby where knowing the ins and outs of everything and the different possible ways of doing things makes it a lot easier. Plus I enjoy it .
Conan, I don't know why you'd think that getting a zoom lens is a waste of money, if you think you'd like the extra zoom capability then why not? Some of the f3.5-6.3 zooms can be of limited use for hand held shooting when the light starts to drop but if the light is good enough to keep the shutter speed up and the ISO reasonable then they can enable you to get good results even if the purists may spot barrel or pincushion distortion in pictures of brick walls. For example I once had a Sigma 28-300mm lens which was by modern standards a pretty poor lens but I took some of my favorite pictures ever with that lens. Plus zoom lenses aren't just for shooting stuff that's a long way off, they can also be used for a different perspective even if you could walk up to the subject and shoot from close distance.
To give you something else to think about as you mentioned mirrorless cameras. I have a first generation Sony A7 which is a 24mp FF mirrorless camera which I think I'm right in saying is the cheapest FF camera you can buy new and used ones crop up at reasonable prices. Ignoring makes and models these mirrorless cameras offer the advantage of being able to use lots of DSLR lenses (but focus performance may vary) and even old manual lenses from the film days. I often use mine with film era primes and have great fun with it. These old lenses can be a cheap way of taking pictures, want a 50mm f1.8 lens for under £20? Whilst action shots at f1.4 will be next to impossible you can shoot hyperfocally or with zone focus and if there's time to deliberately focus on a subject (portrait/scenic/landscape/still life) then all is good. Manual lenses could also sit alongside AF lenses as a cheap way to fill gaps in your lens line up where you don't want to spend a lot. For example if you don't do much wide angle shooting spending hundreds on a modern lens might not make a lot of sense and you might be happy enough with a film era 19mm f3.5 or 24mm f2.8.
Another advantage of mirrorless is being able to auto focus anywhere in the frame. I find this to be a big advantage when taking people shots and face detect means I don't have to move the focus point and can just concentrate on composition and capturing the moment.
Anyway. Just things to think about but if you do end up going FF or even make a sideways step into a different APSC set up maybe you could take a serious look at the mirrorless options.
Tele-Converters.... basically an extra bit of glass you clamp to the camera, then clamp your lens to that.
They used to be 'Cheap Reach', not costing an inordinate amount, and stretching a lens by, typically 1.5x, 2x or 3x the focal length.
I have 2x and 3x converters in the bag of M42 screw-fit kit... I think That I paid packet of fag money for each of them, at the time.. (When folks grumbled that a packet of fags was almost two quid!) These days, with all the electrickery of modern electric lenses and the couplings they have to have to maintain Auto-Focus, seems that TC's are first not so cheap, and oft dont offer so much reach, tending to be 1.4x or 1.7x ish, as well as the historical niggles of potential image degradation and loss of f-number.
For your notion; you are looking at around £150 and up for a new third party converter, around 1.4x. That would ramp an 18-55 to the equivalent of a 25-77. Even a 2x converter will only get you up to 110mm equiv, on the 18-55; its not going to get you into the long reach arena of the £200, 55-200.
There are some good reasons for TC's, and birders have historically loved them; the reach they can get out of a relatively moderate tele, like a 300, pushing that up to maybe 450mm equiv with a TC, and a higher grade 300 lens compensating for the losses a TC brings, can make them a particularly useful way to get a lot of long reach.
It used to be derrigeur at one time to use a shorter perhaps 300mm 'mirror' lens, with a converter, because the overall package was overall shorter and more compact, so easier to swing through trees with, than say a 1000mm mirror alone, and you could wip the TC off, if you wanted to get that much wider, when 'zoom' s weren't so available or common, or even very good, TBH!
But, its not really a solution to anything in your case I don't think. If you want that amount of reach, just get the Nik-Kit 55-200.... and get as much of it for as little, without the hassles!
If you do go legacy lens and adapter for the lols, then you'll likely find a fair few Tele-Converters come up for relative pennies, and you can play to your hearts really, its all lols, but they still aren't a substitute for a proper dedicated long-reach-lens.
You'd be better off getting an 18-135 or 18-250/275 used. Image quality won't be stunning (but will be better than a 2X converter on a kit lens) but it will let you try out a very wide range of focal lengths. After you've used it for a few months you can assess which focal lengths you prefer and buy better lenses accordingly, selling the superzoom at little loss because it was already used.