- Making David a recurring villain
I don't know, that'd be big trouble for the Animorphs. I don't mean just because he can morph, but because he's willing to go to Visser Three/One and spill the beans. Something would have to be slightly altered so that's not such a big possibility, like maybe Visser Three/One wants David just dead, too.
- making Mellisa a more important character(maybe even an animorph) or at least make her one of the few humans who actually knew the animorphs identities(were there any humans who knew in the original series?)
Until they reveal it, I don't think anybody knew (though obviously the former Visser One and a few other Controllers suspected it). I guess after #30, ex-Visser One knew it for sure, and so Marco's mom also knew.
But if Melissa wasn't an Animorph but she knew about the Animorphs, Jake would take her into the group anyway. Another David situation, though more under control since Visser Three/One wouldn't know about her. Still, the Animorphs would know it's too dangerous to just leave her at her house; she'd be stuck holding life-or-death information in a house with two Controllers? No way.
- Give Marco a girlfriend(possibly Mellisa)
Ehhh.
- have a scene where Tom(for a moment) is freed and Jake(and younger brother if he's there) get to tell Tom about the Animorphs and how they'll never give up trying to save him, blah blah blah, and then the all mighty Ellimist puts things back in place so Tom's a controller again and doesn't remember that little convo he had with his brother(s)
I guess that'd be good for Jake, but it just kind of feels, idk, just dumb.
- Make Rachel the Ellimist's right hand woman when she dies
Besides "HURR SHE WAS AN ANIMORPH", what sets her apart from any other (female) Andalite warrior who died fighting the Yeerks?
- Give Cassie a decent battle morph
For god's sake, YES. It was bad enough she only had her horse morph on their first trip into the Yeerk pool. Come on, she was with the rest of the group when they were getting battle morphs. She could practically live at The Gardens. WHY THE DAMN WOLF ALL THE TIME?
- completely redo book 48 but keep the ending (actually scratch that, change the ending so we know what happens to David. I still think Rachel should let him live, only to be scooped up by Tobias who thought he thought was just a normal rat lol)
The Tobias ending would be rather humorous, but I personally think it wouldn't fit the serious nature of the ending of that book. The question of what to do with David was a very humane decision that Rachel had to make, and I think it would hurt her if Tobias just came in and ate David.