I'd totally support a Fate/Big Sister or Fate/Milf Order spin off that had many of the female servants far more mature than their masters.
I mean, we have Fate/Kaleid Liner Prisma Illya which is basically lolicon incarnate so we should have something for the other end of the spectrum.
Well, many of the female servants are already far more mature than their masters. But I understand what you meant, you want current servants with shouta masters. Honestly, I totally back that idea. I would love to see the interactions between Saber x shouta Shirou, Caster x teen Shouichirou, more Extra Rider x shouta Shinji, Extra Saber/Caster/Lancer/Altera x shouta Hakuno/loli Hakuno, and any other female servant/male master I might have missed.
I would also like to see more loli Rin and Tohsaka Sakura too.
God I just finished Shinji's part of Fate/Extra and this reveal destroyed me
Meh, even as a kid he's still a prick. Only as a kid it's a lot more justifiable. He is however, smarter and seemingly more likable to his servants than any other Shinji though. I recommend reading Fate/Extra CCC Fox Tail after playing the game.
I'm still faithfully waiting for someone to translate Fate/Extra Sexy Sakura Sentai so I can finally complete the Fate/Extra series.