Only a few of your points are really on-track IMO . . . like the "less annoying" thing. (I'd use "less obnoxious" in any case)
Anyway: voice actors don't need to be super high quality here. Decent saturday morning cartoon quality acting is absolutely fine. Honestly, directing and writing are WAY more important than the actors themselves too. (look at the difference in voice acting quality between Sonic Adventure 1 and 2, with most of the same voice cast)
A big problem in many games, isn't the characters themselves at all, but the issue of trying to cram too many into a single game (especially trying to tie them to plot points). Sonic Adventure 1 and 2 did this pretty decently IMO, some thing they went to far with it, it worked well enough overall IMO. (and it's important to note that SA2 cut out more characters than it added -both playable and non-playable)
Sonic Heroes totally went in the wrong direction there with a whole mess of characters compiled from their back log of existing cast as well as adding yet more.
The 2D handheld games have less of a problem with this (and the story issues too) given how they tend to be more streamlined and minimalistic in general, not pushing the story as heavily and not super heavily voice acted or cutscene laiden either. (and some story-heavy games like Sonic Battle manage things pretty decently too . . . I can't comment on Dark Brotherhood though, never really bothered looking into that)
The other problem, both in regards to characters and in Sonic games in general is the story: either they make the plot too complex and just end up being stupid and full of holes, or they try to dumb it down but it falls flat, or it's just an inherently bad/weak/unfitting story that's also poorly executed. (Sonic '06 would certainly be the latter IMO)
Sonic Adventure and Sonic Adventure 2 handled things alright for what they were, and IMO Sonic Adventure 2 was a pretty decent evolution of the writing, story telling, and acting quality (needs work, but going in the right direction). Then Heroes kind of throws that out the window for a watered down story thrown in the for the hell of it, and overall premise/situation that doesn't make sense (on many levels) given where Sonic Adventure 2 left off. (especially with Shadow randomly thrown back in with no explanation, and then a bunch of confusing and rather pointless overall dialog and story that didn't hold up well at all compared to the previous games)
Shadow . . . I actually kind of liked, but it still failed big time. I like the premise and the basic idea of the story and game itself, but it needed more polish in gameplay and level design (and graphics), but the story and character interaction are separate problems in and of themselves.
The story itself is an interesting concept IMO, and would have made a decent one to pose as a direct chronological sequel to Sonic Adventure 2 (more so had Sonic Heroes not had Shadow . . . or had Sega made a different game as a direct followon to SA2 -still lacking Shadow).
Anyway, I like the concept of the story, but the execution was pretty weak, not properly fleshing out many of the few areas of story/backstory that's actually interesting, not filling plot holes from the previous games, and quite often contradicting previous games' stories and adding new holes to the plot.
I'm totally fine with the game being dark and gritty like that . . . I liked the bit of darker story elements worked into SA2, and as a spinoff (not intended to be a typical main series sonic game), that would have worked well IMO. But it would still definitely be a matter of good writing and directing (and overall execution) of that story, and having good gameplay on top of that.
Oh, that, and the extremely limited profane vocabulary used in the game was really inane and stupid . . . if they'd actually pushed it up to a Teen rating (as originally planned) and had good writing that actually TASTEFULLY made use of that, it could have been OK. They also could have been totally fine without that too, dark storyline and all (plenty of good cartoons with relatively complex stories getting rather dark at times that worked totally fine within G/PG boundaries ;)).
Really, if the writing/plot, and acting/directing was on level reasonably comparable to decent (let alone good) animated TV show production quality, there'd be a LOT less to complain about in these games, and much more chance for the story to actually enhance the gameplay experience overall. (at least for those who actually appreciate a story and get more immersed in the game through that -which I certainly do)
And they were moving in that direction with the improvements from SA1 to SA2 . . . it just fell apart after that. (even more interesting since SA2 apparently had a relatively low budget and short development time compared to SA1 and other typical mainstream heavy-hitter titles of the time)
