Read a couple pages on this topic and wanted to toss my couple cents in.
The idea of making minions tougher at higher levels has some merit, I feel, but ONLY if you reduce the number of them in the same encounter.
Otherwise, all it is is a 'masked' method of forcing grouping.
I don't want to be able to tank 2 +4 bosses at the same time, at least not solo. Grouped? Yes. I should be able to TANK them. Kill them? Heck no, that's what the scrappers and blasters are for =)
But I digress...
If one of the game concepts was supposed to be "feeling like a superhero", then you SHOULD be able to handle more "even" minions as you increase in level. Or else provide incentive to fight lower-level minions.
Yes. Those even minions are now higher level than the ones you fought before. That's fine and good from the "real world" perspective. But from the GAME perspective, you're talking about removing almost ALL sense of increased power as you level from the game.
Why level if the fights are all going to be the same scale?
If you turn the game into the same scale of combat throughout the level span of a character, you're increasing the grind factor, reducing the 'I'm a stronger hero now than I was before" factor, and just changing the feel of the game, when the feel is the most important part for me.
Increase the HPs and difficulty for higher-level minions, fine. But I think, as has been suggested, that a better approach would be to scale up the encounters by adding an intermediate level between Boss and Archvillain, and then increasing the number of LTs and Bosses out there.
Or replace the current minions with "Lackeys", make Minions tougher, somewhere between current minions and LTs.
Anyway. My couple cents. If the most you should be able to fight is 3 even minions, or 2 +1 LTs, or 1 +2 boss, then you'd better also rescale the missions so that I don't run into 6 even minions, 4 +1 LTs, and a +2 boss all in the same 10' x 10' room on a solo mission.