They're backed into a corner now.
On one hand, they could easily just give Bryan the win at Fast Lane and have Brock vs. Bryan. That would be the biggest underdog story in the history of wrestling and I would love every, single second of it. Bryan wins, and it's a huge deal in wrestling.
If anyone but Bryan faces Lesnar at Mania, after this announcement, forget about it. WWE can't seriously think it's a good idea to feed Bryan to Reigns at Fast Lane, right? I can imagine Vince thinking that could work, when in reality, no fucking way. That would be worse for Reigns than losing to Bryan and working a match against Swagger or some other bum.
I suppose they could turn Reigns heel, but then Lesnar would be a face. Lesnar's popular enough, but I can't imagine just watching him tear Reigns apart at Mania will be worth the viewing time.
Who knows. Just give Bryan the main event and move on.