read heavens feel.
Spoilers*
Rin goes looking for Touko Aozaki to make a better body for shirou so he can do shit again.
*End Spoilers
If it can be accepted that the KnK universe is the same universe as the f/sn universe, and the KnK universe is the same universe as the tsukihime universe, then by the transitive property of association, the tsukihime universe is the same universe as the f/sn universe.
Its not a matter of them being in different universes that makes them impossible to connect at the moment. its the fact that they literally take place at different points of time. As for the law of there only being one person with the mystic eyes of death perception at a time on earth, Ryougi's eyes are technically not the "mystic eyes of death perception". That's a mis-diagnosis by Touko that she came up with based on the very little information Ryougi gave her. Ryougi's eyes are the "Eyes of Origin/Root/Akasha, something completely different. (just similar in appearance.) Read the lore thread if you want the full explanation.
So really, all nasu has to do is make the next Melty blood game take place a few years later, use some new bullshit excuse with the tatari, and he could theoretically throw servant's into the mix. Should he? FUCK NO. Melty Blood is supposed to be about vampires, darkness, murder, and shit. Not ancient heroes duking it out.
IMO, if people want servants, go play fucking unlimited codes.
and dont get me started on a character vs character discussion. this isnt the place. >_>