smashkirby
Smash Hero
Oh, yeah. As big as Diddy was at that point in (modern) DK's lifespan (Not to mention how Dixie got a trophy, yet he DIDN'T?)? I've always been of the belief that Diddy was TOTALLY considered for Melee as a playable character (and was even close enough to the point that he would have had a set of fighter trophies for himself), but Microsoft's buyout of Rare muddied up those plans, as Nintendo (and especially Sakurai and HAL Labs) wasn't all too sure as to whether or not they would retain the rights to Diddy (and everything else that Rare created for the DK series up to that point in time.)This wasn't exactly clear during the development time of Melee. The Rare buyout just happened and Rare was actually trying to claim the rights to Donkey Kong, and if they couldn't, at least their own created Kong characters of which of course Diddy.
The weirdest thing about Melee was the lack of a Diddy trophy, but Dixie and K.Rool where there. This leads me to believe Diddy was probably planned at some point. Another suspicious detail was Event Match 2 where you fight a Giant Donkey Kong and a small Donkey Kong in the red costume. Might have originally been meant to be Diddy. This is of course a theory. But I don't think am far off, as Sakurai also stated with the Diddy reveal in Brawl that his arrival was late. It's illogical that he'd been one of the planned fighters of Smash 64, that means Melee was probably the "right time" for him to join.
As you've said, a lot of DK's content in Melee was also very odd in regards to single-player content. Namely Adventure Mode and Event Mode, where it legit feels as though the mini-DKs that show up at various points in Melee were meant to be roles filled by Diddy himself.