I can tell you right now, MC15 is never going to NATIVELY play a blu-ray with full menus and whatnot without some other software. To do that, it would have to become a licensed player, and that is not a fun thing for anyone. The plugin idea is a good one....passing a disc off to the 10' TMT3 UI seamlessly. That should be pretty simple in reality because all you're doing is calling uMCEDVDPlayer.exe with the drive letter. From there the 10' UI takes over for disc playback. That's all 7MC is doing under the covers anyway. There's no reason MC15 couldn't do the same thing with PDVD, TMT, or even WinDVD.
Now, if you want to throw something like AnyDVD into the mix to get around the AACS LA requirements, MC15 could integrate libbluray that's being developed as open source for BD-J and menus. With ffdshow for dxva/subs/bitstreaming, that'd be a pretty decent solution, but, again, the disc would have to be unprotected. And for those that want to decode rather than bitstream, you're going to run head first into the missing DTS-HD MA decoder problem. You can decode DTS core, but, there is no decoder available for DTS-HD MA. Nonetheless, this is certainly doable with a great deal of effort. I also do not have any idea where libbluray is in their development process. (Meaning how far along they are). Given how long SlyPlayer has been in development, I know how much of a challenge it is to create a player that can do all that.
Anyway, food for thought. I don't disagree that it would be nice, but, I think option 1 is the most viable for now and should be extremely easy to integrate.