Yes, Val Kilmer is on board for Top Gun 2. No Coppola and Hackman AREN'T involved!
We already knew Top Gun 2 was coming and that Tom Cruise and Jerry Bruckheimer would be involved. However a Facebook post by Val Kilmer send the wider press on a wild goose chase with claims he'd confirmed that Francis Ford Coppola was directing and Gene Hackman would star!
Neither of these is the case! As later clarified by Kilmer, they were examples of when you'd say yes without seeing the script; and the fact that Cruise and Bruckheimer were already onboard was enough to convince him to join the cast.
It IS disappointing that so many news sources jumped to conclusions without investigating first - and some of the massive factual errors (The Guardian claiming Bruckheimer was STARRING in the film) showed little sign of actual film knowledge.