Wow. First post in a LOOOONG time.
Sequel, definately.
While I can't argue that there were disappointing elements of the first movie that I wish they'd erased entirely (a list of which I may just turn into an entire post...why not come back with a bang), the biggest complaint from most people was that Lee's emphasis on the origin, setup, and drama ate up most of the screen time and never seemed to build towards the big payoff. That being said, superhero sequels have the benefit of letting you get to the good stuff quicker, and I'd rather not invite people's impression of another movie to be "oh...good...MORE of how he became the Hulk"
I'm not a pro here, but it isn't rocket science:
Have the Hulk wrecking something in the opening sequence
Calm him back down and setup Banner off living in wherever he's living
Betty's trying to help him remotely, i.e. sparse comunication, etc.
Emil Blonsky (anybody else feel like a script writer is probably going to change his name?) exposes himself to gamma radiation
Bruce's "cure" isn't complete and depowers him into the grey Hulk
Blonsky transforms, wrecks stuff, can't change back
Decides to kidnap Betty to force Banner out of hiding to cure him
Banner shows up as the Grey Hulk, fights and loses
Abomination leaps away with Betty.
Ross revives Banner, makes a deal with him
Banner amps up to Green Hulk (i.e. "Hulk is angry...")
[20min. - 2 hour] Hulk/Abomination super fight
Betty rescued, Ross backs out of deal
Banner escapes (have him hitchhiking down the road, g*da*it! Haha.)
Or something like that.