Lots of things were disappointing in Sonic 4. I remember they had this meeting where they(Sega) invited Sonic fans and asked them what they could improve on, if they made a second one(Episode 2). Everything brought up in the meeting, I agreed with:
1. Fix the physics.
2. Songs were great, but the synthing should be BETTER.
3. No FUCKING bottomless pits at the end of each act.
4. No act specific gimmicks.
5. No recycled stages from earlier Sonic games.
The last one is funny, because Casino Street is basically a pre-rendered version of Casion Night, Lost Labyrinth is basically a pre-rendered version of Labyrinth and Mad Gear is basically a pre-rendered version of Metropolis. C'mon, do something that isn't the same, just prettier. I excuse Splash Hill because it the Hill/Forest zones are pretty much traditional starter zones in the early Sonic games, though Splash Hill could've used more, well, SPLASHING(should've been more water oriented). After going to the movies recently, I had an idea of a new zone. Cinemapolis(or Cinema City) Zone, which is modeled after classic Hollywood, buildings addorn in neon marquees, roads that are giant flim strips, film reel loop-de-loops, giant drive-in screens on top of buildings showing movies of sonic running around, and indoor areas that look like theater lobbies and hallways adorn with movie posters. Sega, it's NOT THAT HARD.
That being said, I AM looking forward to Sonic Generations. Classic gameplay looks more like what Sonic 4 couldn't do.
*notes this all down for her upcoming Sonic 20th Anniversary Video Review*
Back on topic, though....
With Castlevania, I think the most basic way for fans to get the attention of Konami is to universally agree on ONE method of communication (Twitter campaign, Petition, Fan-Game Model, etc.) and have everyone stick with it. Of course, this brings up issues, because not everyone thinks the same and everyone has their own opinions. That's usually why some things are started by one or two people, and then people either jump on the wagon later or don't. This creates less initial friction and gets the ball rolling quicker. However, again, the ball only continues rolling and picking up speed if the fan community really adheres to it.
And again.....there's never any guarantee that Konami would even give a shit. So going into things, that would need to be one thing that is always kept in mind. If nothing else, however, it would prove that the fandom gives enough of a shit OURSELVES to try.