For the powergamers, I wouldn't worry about them making combat wombats since it is VERY hard to make an unbalance/overpowered character in Fate and as Fred has already mentioned generally in Fate the flawed characters are the powerful ones (probably the best way to be a munchkin in Fate is to be a fishmalk and try to milk Fate points by doing random crazy stuff all the time, but that can be reined in pretty easily by a good GM).
With the powergamers I'd try to steer them towards aspects that are awesome but can still be compelled. For example:
Looks like a badass: good for many things but can be compelled to make people notice him and focus on him when he's trying to maintain a low profile.
I don't take any lip: quite a bit like Dresden complete lack of diplomatic abilities (at least in the earlier books) where he needlessly antagonizes all kinds of people.
I can take him alone: gets a compel to take a baddie on one-on-one.
That sort of thing, basically give them compels to do muscleheaded stuff and they'll get Fate points for acting "badass" and they might even like the game. Probably cater to what they want a bit and give them plenty of jerky enemies that disrespect them personally that they can pound into the floor. A bit of mindless violence can be fun and the aspects could get them in the mood of comic book style smackdown, which seems to be what they want.
Nice.
The flip side of this is to go a different route: to make Failure either Funny or Cool, while still giving out extra EXP or Fate/Luck points as a reward.
The old RPG
TOON is a great example of this. Everyone makes their own version of a favorite Cartoon Character.
A mature group can even mix Heroes with Villains, so that the FilmMaster does not have to do all of that work.
(If I can get a mature Group, I'll play the Mega-Villain with some heroic support staff [NPCs] to get the PCs out of really deep trouble.)
The idea is to get your TOON PC into as many difficult situations as possible, and see what happens.
Remember cartoons like the Original
Merry Melodies (Bugs Bunny, Tweety and gang?) or
Tom & Jerry or the fairly recent movie of
Who Framed Roger Rabbit?
Sylvester chasing
Tweety and always failing in spectacular ways:
sometimes it's getting munched by
Spike the Dog: sometimes sliding into the Ocean: sometimes having Jerry throw a frying pan into is face at the last moment.
The same thing happens to
Tom Cat, with
Jerry D Mouse and
Butch The Dog.
The goal is to be able to make it to the end of each Game Session and look back and everyone can compair notes and laugh their heads off at what all happened, even to their Character.
Once they see that 'failing' can be fun, try to get more serious and step up to more serious Cartoons:
Thundercats,
Silverhawks,
Biker Mice From Mars,
Teenage Mutant Ninja Turtles, etc.
(In
TMNT- The Filmaster would be
Shredder, and the Villain PCs could be like
Bepop,
Rocksteady, or one of the
Foot Clan Ninjas.)
If they are still "Game" - give the
Dresden RPG a try.
Remember, Mature Players are made, not born. Every GameMaster started as a Munckin (not only a little kid, but also the RPG terms) of one kind or another.
Also, get out there and start looking for more Players.
Yes, it takes time, and it can be a pain in the butt - however, it can pay off.