I'm still feeling the pain of watching MAC and ME (1988) last week. If I remade it :
Better animatronic effects for the aliens. Better plotting and characterisation - the family could have just lost the father to illness (instead of not explaining his absence). Mac has empathic abilities , he hones in on the boy's pain and identifies with his loss and THAT is why he sneaks into his car (not a goofy cartoony sequence on the highway).
Maybe have a sympathetic government agent guy (and not a clumsy cartoon buffoon) who explains to the boy that they could maybe use Mac's abilities to aid mankind and cure illnesses like the one that took his dad..
Megaforce (1982) - the team could use iron man type exo-skeletons (each with different abilities) that have a symbiotic connection with their vehicles. Show them actually saving women and children from dictators instead of just driving around the desert blasting away at nothing...
Superman IV : an animated DVD of this would be cool. Set it during a Trump/ N. Korea stand off. Supes destroys the nukes but is branded a dangerous traitor. Have Bizarro as they originally intended and fix China's Great wall with super speed NOT with magic eye powers!
What films would YOU fix?