Unfortunately, Universal only seems to be interested in "fresh" IPs, which they seem to define as a major tv/film released within a few years of the attraction. BTTF is one of my favorite films (and rides), but as others have said above Zemeckis definitely won't let a new film get made (and I prefer it that way). I would LOVE to see BTTF rebooted as an attraction though, but I only see two realistic ways that would ever happen: 1. if somewhere down the line new execs were hired who are open to the idea of using old IPs instead of modern ones, or 2. if a tv show/series were to be made (say by Netflix or something). There is potential Zemeckis would be open to a series rather than a film (there was already the animated one), but I wouldn't hold my breath.
But if it were to get made, I love your idea of a crossover in style between Indiana Jones and Forbidden Journey. Like it starts out with an Indiana Jones style, driving through modern (80's) streets of Hill Valley, then all of a sudden you're whisked away into the air (a la Forbidden Journey) and into a different time-frame. Man, that would be so cool!