comeback mechanics are basically only good for hype, and hype is perhaps one of the worst reasons to introduce a mechanic IMO. fighting games are doing this way too much these days. x-factor in marvel, revenge meter in street fighter... it's really silly. is there a problem with being in an advantageous situation because you outplayed your opponent? no. i think if you need a comeback mechanic to make your game hype enough, you should probably go back to the drawing board on the game as a whole.
I'm not going to comment on the overall quality of Brawl since I don't want to get into that. What I will say is that
I think that Brawl is plenty hype, and that Lucario's Aura, as a singular mechanic, is in now way a sign that the Brawl team needed to go "back to the drawing board". Lucario minus Aura is a perfectly good character, and Aura is just an interesting subversion of Smash's general mechanics.
look at this comeback in mvc2. there's no mechanic in that game that allows justin wong to make this comeback specifically because he's behind, and yet he still does it and it's incredibly hype. he makes the comeback because of a few reasons:
1. he starts off with a lot of meter, which he's gained over the course of the match.
2. he outplays his opponent in numerous scenarios
3. the game's mechanics reward justin significantly for outplaying his opponent in those scenarios. each time he hits his opponent, he inflicts at least 1/3rd of the victim character's HP.
I've seen that clip before. That clip is a great clip and a horrible clip at the same time, and here's why: slippery slope (and I don't mean the argument). It's been well known in game design for a long time that slippery slope, or the tendency for a player to do exponentially worse with each mistake made, is bad design, or at least suboptimal design. Now, team based fighters have always had to combat this because slippery slope is a natural part of the mechanics (one less team member means less damage output, less options, etc., and each action taken away from the player makes not just comebacks but even basic play that much harder). This is the real reason, at the end of the day, why X-factor is such an effective mechanic in UMvC3; not only does X-factor in and of itself mitigate (but not eliminate, which is important here) the effects of slippery slope, but it's even proportional to how much the player needs it! We may quibble about the particulars of individual character balance, but X-factor, in and of itself, is a mechanic that adds LOTS of tactical depth for VERY little technical overhead cost.
Watch that linked match again, and this time think of what would have happened had that been anyone other than Wong playing (that Wong factor is notorious). Even in the case of many top level players, that would have been a non-hype, non-fun, totally uninteresting match; even the commentators say it after Storm dies ("Now we have a real match!"). Remember that, even though top level play is the culmination of a fighter's mechanics and design, it DOES have to cater to and be balanced around lower levels of play, as well. In a game like Mahvel, one misplaced assist call could place a player in a 1 v 3 situation, and that's a HEAVY cost for a simple assist call. Comeback mechanics help to mitigate those kind of extreme circumstances.
That's a lot of analysis that will come in handy in a second.
anyway the problem with lucario's aura isn't that it rewards players who purposefully do badly. lucario's optimal play almost never includes taking damage on purpose (except to prevent like, falco chaingrabs because of how absolutely awesome stale moves function in brawl). the problem is, the character is better at higher percents than he is at lower percents. lucario's aura changes the nature of what a "lead" entails, and lucario's effectiveness is very artificial because of it. additionally, lucario is not rewarded for AVOIDING getting hit the same way that other characters are because of how he's rewarded FOR getting hit. conversely, he's rewarded for having good DI and surviving a long time in a way that's more than just "you aren't dead" like with the rest of the cast. this skews the skillset lucario players are tested under to survivability than other things like general spacing to avoid getting hit and things of that nature.
And, even though all of the mechanical observations you've made are correct, all of your judgments are off base. First of all, you're entirely correct that Aura changes the definition of a lead in Brawl... which is the point. That's why it's a mechanical subversion, and that's why it's compelling; fighting Lucario is markedly different that fighting any other character for this reason, and that's a good thing. Your analysis that Lucario is not rewarded for avoiding hits equitably is off, as well, because he IS rewarded in exactly the same way that other characters are (that is to say, he's not punished for avoiding attacks). Where you've missed the mark is that Aura isn't an
alteration of existing reward structures in relation to whiffed attacks; he simply has an
additional reward structure in place for getting hit that other characters don't have.
Of course, I don't see this as a bad thing because Lucario's Aura is designed to be a subversive mechanic, one that turns established Smash mechanics on their head, and in that regard, it performs wonderfully. Lucario is a wonderful beginners character in Brawl because he's a bit slower, has low-complexity strings and combos, and Aura lessens the blow of taking damage, thus mitigating potential slippery slope. Lucario is an equalizing character, one that excels in lower levels of play and plateaus out at high level, which is exactly what you'd want from a character with a mechanic like Aura; I'd argue that Lucario is actually the most well balanced / designed character in Brawl for this reason.
Unlike your assessment, I don't think that the altered skillset testing that is done to Lucario mains through the Aura mechanic is any more significant or worse, normatively, than, for instance, the fact that Diddy mains are more harshly tested in terms of item control, or Mario mains in terms of wall jumping; different is not bad. In addition, the ways in which Aura change the dynamic between players at high damage levels in intrinsically compelling; the same way that X-factor prevents a player in the lead from resting during a match (or the same way Wong factor
should have prevented his opponent from resting in the above linked video), Aura allows for a dynamism in matches that other characters just can't match.
Aura is not a bad mechanic. It's not even badly implemented. It's different, but that's a good thing in a fighter like Smash, one that relies on subverting fighting tropes in order to progress the genre in interesting ways.