I don't think a Timon and Pumbaa movie is without merit, so I'd have liked a much better one. I'd start off when Timon and Pumbaa were babies, because I had these as a kid and they were adorable:
And just have the movie be about Timon and Pumbaa's lives; the troubles Timon faced in his colony and Pumbaa in his sounder, and it would end when Timon and Pumbaa met, or perhaps ending on the "One more run won't change our lives!" line, because the audience knows that this is where they came into the original Lion King. It would be a more serious film than 1 1/2 was, with none of the awkward movie-watching, none of the awful "Timon and Pumbaa were in the background of this scene and this scene" stuff; just a straight biopic on Timon and Pumbaa. Less serious than Lion Kings 1 and 2 because let's face it, it's T&P, but not purely comedic as 1 1/2 tried to be. With perhaps some shots of T&P with cub Simba alongside the credits, and the snail-eating-contest as an after-credits scene (I like that scene). I also wouldn't call it "Lion King 1 1/2" but perhaps something like "The Tale of Timon and Pumbaa". Sounds cliche but "LK 1 1/2" is a weird title to me.
Of course, a prequel showing Mufasa and Scar's cubhood, or a sequel to SP, are things I'd rather see. But I still feel a T&P film has the chance to be good, and they definitely dropped the ball on 1 1/2.