Excellent Adventure is a real classic and Bogus Journey wasn't too bad for a sequel. A third IS doable, I think, with a decent story.Focussing on their kiRAB might be the way to go.
The way I would do it is that now they're in their forties and none of their promised future has happened. They're not in the band, Ted has a 'proper' job and is treating his son just like his dad used to treat him, threatening to send him off to military school. Bill is an alcoholic loser who bores people with stories of his and Bill's adventures (which no-one believes, obviously) and how he's gonna bring the world together with music etc.His teenage daughter is a really talented singer but her dreams keep being ruined by her dad's behaviour...
The long and short of it, Ted's son (Ted Jr.) and Bill's daughter (Billie? lol) somehow get whisked away in the phone booth and Bill and Ted have to go save them. Instead of historical figures we see them meeting old stars like Elvis, Sinatra, Jimi Hendrix etc. They have to get back in time for Billie's big audition where she is backed by those stars. Pretty rubbish but at least one idea...