There is a plan in the works to make a Batman Beyond movie. It's one of those vague projects that has no substance to it yet, but maybe by the time the series takes place, the movie will start filming.
Kevin Conroy isn't old enough to play Elderly Wayne without lots of prosthetics and make-up, but he has the perfect voice. Michael Keaton probably wouldn't portray elderly Wayne the way we'd want. He made a good mysterious and antisocial Batman, but I doubt he'd be a satisfactory grizzled curmudgeon who looks like Wayne.
As for Terry, I know that Joshua Jackson (the one decent actor from "Dawson's Creek" and "The Skulls"馃槈 was/is being considered. I'm not sure if he's too old now, but a few years ago, he would have gotten my vote. He has played a hot-headed ne'er-do-well before, and I think he'd be a good Terry McGillis.
I just got this funny image of Anthony Hopkins as a grizzled, old Batman. Anyone see "Mask of Zorro"? Not a suggestion, just a funny image.