So, are you thinking accuracy/damage buff? Or a defensive property? Or fighter powers that say 'If versatile and used in two hands:' ...which could be a cool subset of powers. I'm hoping for a few feats to make it a bit more perky.
Ideally, it would be a fighter build option like the tempest fighter, and would include a basic set of starting bonuses coupled with powers that read things like "If versatile and wielded alone." Or something better phrased than that, but you get the idea. The best way to do it would be to come up with some kind of theme to give to versatile weapons wielded solo (ie, not with a shield or another weapon). You know how shields have all kinds of pushing abilities? Finding a theme like that would be the best way.
I'd suggest grabs and acrobatics as the theme. Maybe allow a versatile fighter to choose acrobatics as a class skill, give him a bonus to attack when fighting with a versatile weapon, an extra bonus to damage when wielding his versatile weapon in two hands, and then create a number of powers that begin with making grab attacks and then follow up with abusing your grabbed opponent in some way.
That's just an idea, there are other ways it could be done of course.
It would also require clarifying how one switches one's grip on a weapon in terms of the rules. But that's needed anyways.
Like... more universal feats? Sort of how weapon focus is good for any melee combatant, not just martial power source people?
I meant more powers, but yes, that would work too.
There are a lot of classes that fight by hitting things with weapons, but which aren't actually martial classes. Martial skill seems like the sort of thing that shouldn't be too hard for, say, a paladin, to acquire.