I made some suggestions about the AoE of shadow priest in another post: My suggested Shadow Priest changes for 9.1.5
I think it would work well but would need some tweaks, I talked about it in another post and the “eternal nightmare” talent might have to be changed so its not too powerful in large pulls, eg: Searing nightmare refreshes mind sear cast, searing nightmare does 33% less damage, searing nightmare shadows do 33% more damage (or generate more insanity for more SN casts). This way you don’t nerf the small pulls but don’t make the large pulls too op.
I don’t really have a problem with searing nightmare having to be cast when using mind sear, seeing as you’ll probably want to be using mind sear to generate insanity anyway, so I don’t see that it really changes the rotation that much to let you do it outside of mind sear. You’ll still be going “mind sear>searing nightmare>mind sear”, the only difference is you won’t have that initial GCD from mind sear. But you know they could solve that by just making the first cast of searing nightmare in a mind sear cast ignore the GCD.
It’d also be nice to have something which allows us to use mind blast and vampiric touch on the move, like the shaman ability Spiritwalker’s Grace. That would be so useful in bosses like The Eye.
Another thing that might be nice, although possibly a little OP, is to make shadowflame prism also proc off searing nightmare, so it synergises well with AoE.