Sony has really opened up the doors for Nikon and Canon users to consider it as a viable replacement, the price point is really aggressive in terms of spec etc.
The Sony lenses can be expensive so its wise to price up the entire system if you are going to replace like for like and then see where you stand and if its worth the cost to change.
Sigma have announced FE mount Art lenses but they are pretty big.
Tamrom are starting to develop FE lenses with their first being the FE 28-75mm f2.8
Zeiss make the Batis range for the FE mount, they are great.
Samyang make FE lenses too.