Well first off we didn't have enemies for 64, and second off, Melee's enemies are kind of what I've been saying in that we won't really get all too much of them if we only have ones taken from the series. Why would there really be a need to make so many different types when they only show up on one level? Actually, the fact that Melee's Adventure mode sort of broke down there after its first few levels into just more and more random fighting could have been a result of this, though there could easily be other explanations going on there. Still, if I may restate a point, if you go with the classic enemies route, you're only going to get quite a few. Thus I don't think there will be a problem with a Morblin Archer and a FE archer, becuase you're probably only going to get one and not the other.
Also, that's a redesign, not a hybrid. As long as the source parts of the hybrid things stayed close to their components, I can't think that too many people would complain. Plus as I said, its a way to get more obscure villains or creatures in there that wouldn't normally even get a chance. For instance you could throw in a Bounder (Those pink flying things from the Kirby series) with the wings of the Ice Climber Albatross, and you've got a enemy that's not really as offensive as a Death Note redesign, as well as a refference to two enemies that likely wouldn't have made it in before.
And the thing with your examples of the repeating things still only happen in about 2 or 3 variations most of the time. Thus you'll get like 2 or 3 versions of the slow/easy to deal with enemies, and exclude the rest.
No, I'm not saying we WILL get all sorts of variety in enemies. I'm saying that enemies wouldn't necessarily be excluded for being similar, especially if we're defining them by their most basic functions (tackling, shooting, etc.) or attributes (big/small, fast/slow). You'd have to have zero creativity to be unable to make them distinct.
And I really doubt most of these enemies would appear in one stage and that'd be it. None of the enemies in Brawl did that (and that's with 46 different enemies). Whether it means an endgame stage that mixes everything together or just seeing the occasional tie-in now and then doesn't matter.
The biggest thing you're ignoring is the need for more content from the source games. Melee had a little bit of this but people wanted more. Brawl tried to focus on original content and people hated it. I know you want to try to justify function over form, but that's not what people want.
As for your hybrids in comparison to poor Grant: it doesn't matter if it's a hybrid or a redesign, it's still too different from the original for people to like. No one's gonna like a Bronto Burt with Condor wings (as if people will see the wings and go "oh yeah, Ice Climber!") or a Space Pirate Kremling or whatever. Maybe another project with Warren Spector, but not here. People want the source content as it was.
Good thing spliced with good thing does not equal better thing. It's almost invariably crap. If you really wanted to squeeze in some of the lesser known stuff, you'd be better off using them in backrounds or part of the stage or something.
It seems though that I must re-establish something here that these limits I believe will be imposed are not based around any form of merit that the random enemies have, or anything to do with them being acceptable or unacceptable, but rather that its simply too much work for too little pay off. You can't really expect that developers will really bother with giving each world their own unique enemies, do you? Maybe in some type of perfect fan version of the game it would be a option, but based on what would likely happen in the real world it just seems like too much.
Melee had 8 enemies from three series. Brawl had 46 enemies, not including the three from King Dedede's side B (there were also a few cut enemies in the data files, at least Blade Knight, Bronto Burt, and Bonkers).
Keep in mind they completely made up 37 of those enemies from scratch. How is that better than using existing stuff again?
There's also the point that I brought up before that original enemies are needed to fill in roles that already existing enemies can't fulfil as obsticles. (Those fire/ice/lightning things in SSE for instance, or those replicating balls. What enemy from another series would really take their place?)
Hotheads, Bubbles, Zoomers, the electric balls from DK Jr., I'm sure there's more for "wall huggers." And for "self-replicators" we have Fusion Metroids, all those Zelda slimes, any mobile plant monster from Mother, probably other stuff I'm forgetting. Zoomers and Zelda slimes in particular had no reason not to be there.
Here's a better question: what are the Subspace equivalents to ReDeads and Like Likes? What kind of tall, durable enemy slowly shambles towards you and eventually grabs/eats you?
At any rate, you're still more concerned with function over content. Who says we need wall huggers or self-replicators? Do you really think people care more about filling generic enemy archetypes over using enemies people know and love?
Aside from being isolated to Adventure mode (unless they expand on the enemies like how Melee would sometimes have Goombas in crates), it's really no better than the old "we need more females/villians/mages/whatever" argument.