There are 2 things that bother me about nip-46. 

1. If we are going to have the added rtl of relaying encrypted messages, why cant we just contact signing servers directly? Add a signing server url to the nip-05 spec, then perform a real oauth2 sign-in, and communicate with the signing server directly? In fact there would be no need to contact relays at all here. In signer devs just need to build HTTP servers, and don't have to fuss with relay and maintaining connections etc. We can even use another layer of encryption if we want to. I'm aware of some of the security tradeoff with exposing a signing server, but I believe it's worth reducing complexity for. 

2. It's still using nip-04 even though we all agree its deprecated at best.