I feel like the original trilogy is so magical that if they were to make a new BTTF movie, I'd prefer it to be a reboot with possible guest appearances by Lloyd/Fox, maybe showing up as cameos. I love them in the BTTF movies, but the trilogy just works so well and ends on a nice note. I feel like seeing an almost 80 year old Doc Brown or seeing Marty in his 50s doing the whole "Thats heavy Doc" might do more harm than good.
There's definitely more interesting time eras you could explore in a BTTF movie, but what I always liked was that the time travels were connected to the characters somehow, and that relatives of them appeared, like Marty meeting his young parents, or us seeing him older in the future. I believe Zemeckis has said that BTTF stays untouched as long as he is alive and breathing, but if he eventually dies and can't fight that battle anymore, the studio will definitely try to push it.