There is absolutely no reason to remake any of these films.
The future in them is still futureistic, the characters still come across as fresh, there is nothing too 80s about them that make them need freshening up.
My parents love them (40s) I love them (20s) and My little brother loves them 9. I'm also pretty sure the younger of my grans loves them too and shes almost 60 so I'm struggling to think of any possible reason for them to be remade other than studios making money and if that was their sole purpose I wouldn't complain if they remastered the films, perhaps adding a couple of minutes of deleted scenes, and then 3Dified them, not something I'd particularly be desparate for, but if they did insist on making money out of BTTF, I'd much rather that than Zac Efron trying to be Marty McFly.