1. The attraction would have a theme or concept that he's not pleased with (e.g. something like Woody's Roundup, introducing more toons into a land steeped in realism or maybe a raft ride which he doesn't seem pleased with conceptually) or
A land steeped in realism ... that has a singing-bear hoedown, a ride on a log past talking bunnies, bears, and foxes as they chase each other past other talking critters, and an entire island based on a fictional character.
Yes, the architecture is intended to be derivative of the "real old West," but very little of Frontierland is historically accurate. Liberty Square make a push for that, but it's clearly all idealized (and has a haunted house with ghosts).
While a Woody's Roundup attraction will be terribly foolish, a "character-based" attraction that falls in line with Splash or Country Bears wouldn't necessarily be a misstep, in my opinion. And honestly, I don't see Disney putting Pixar in Frontierland, especially with a Pixar expansion coming to DHS.
I respect Marni's posts, but even with his insider knowledge, I think it would be hard to judge just yet what kind of tenor the project will take on. I think it's just far too early in the process for judgments to be made. Things that sound stupid sometimes turn out amazing (Splash Mountain doesn't conceptually sound like a fit for Frontierland, but now it's beloved).
2. It would change the setup of Frontierland in a significant way. e.g. diverting the Liberty Belle to only travelling around the smaller southern island, resulting in a shorter ride there.
Personally, I'd be okay with things like that depending on what is actually produced.
I'd actually be okay with it, too. It would be a shame to lose the kinetics of the Liberty Belle, so I hope they at least keep it in some functional capacity, but the loss of the "back end" of the Rivers wouldn't completely bother me. And I don't see them removing the river altogether, so I doubt that's even on the table.
Actually, a bridge to the "back part" of the island would be a welcome addition. It would add accessibility to an otherwise less-traveled area of MK. It could also result in some new things to see and experience from the Railroad (think along the lines of the town of Tumbleweed, along with views of the new attraction).
I rather like the 7DMT which was endlessly derided by many on here during construction. Now, it's not an E-ticket, but it's a solid addition yet some criticized it to such an extent that it seemed like a worthless addition. To each their own, though.
It's a great looking ride, adds atmosphere and energy to that area, and it doesn't limit age groups. Yeah, it's a bit short, but it's still a beautifully themed attraction.