Also, as much as I like MoI, as I said it seems like the better creators were working on other projects, and that most of the coolest stuff about MoI was more of an accident.
Well... is there a quick fix for the soulborn?
I think a quick fix for both Totemist and Incarnate is to give them full BAB. That pushes them a bit. And then you basically can the soulborn - it's totally redundant.
The other fixes that are necessary are for the crappy avatars. They suck compared to so many other binds, when they should not only look cool, but actually rock.
I'm not great at plotting appropriate power levels, I tend to balance UP and let more experienced players tell me when I've gone too far or not far enough.
Full BAB would be fine.
Sinfire Titan said in his handbook that giving Totemists Magical Beast HD would be a nice and easy patch, and it does look pretty good. That's probably the easiest one to houserule.
More BAB would also be nice for the Incarnate, but I'd also want more class skills and skills per level, as well as doing away with the aligned soulmelds restriction and general focus on alignment (D&D's focus on alignment is lame imo, most parties are required to ignore a lot of alignment stuff just to function). Basically the Incarnate needs a bigger band-aid than the Totemist.
The Soulborn is redundant, as you said, but its focus is clearly supposed to be combat. A fix that made it more unique would be nice. Hell just a few more unique class features would be a giant bonus. Letting it switch its essentia around as a free action would spare it the swift action every round, and maybe some kind of kicker on its smites. Really though, no one smart is going to play the Soulborn without a fix anyway, so until someone gets serious about a fix no one is going to get serious about playing it, and most groups can scrap it as you say.
As for the Avatars, my first use of MoI taught me not to sneeze at small bonuses which you can build up and accumulate into something formidable. That's how meldshapers get their groove on, by cherry picking small bonuses from multiple soulmelds to achieve a tailored attack routine. They COULD be better for sure, but they're not garbage now.