I'm not saying Revan would make an appearance. Only that, they did at one point think it. They had a model and voice ready. They just never used it - due to Lucas I believe.
No... it wouldn't happen. Maybe you saw a Mandalorian Helmet that looked like Revan's. It just wouldn't happen, it would break the lines of canon and it would make things weird.
He couldn't have been a spirit, because the longest we've seen one last is about 300 years.
Plus I think GL doesn't evne know of Revan's existence, and if he did he'd probably retconn Revan. Because quite frankly he wouldn't like Revan, I don't. Well I use to like him till after that horrendous book.