the new Hobbit set common Gandalf has this
I AM GANDALF, AND GANDALF MEANS ME!: As you reveal your force, you may choose 1 keyword possessed by another character on your force. Gandalf possesses that keyword this game in addition to his other keywords.

DC10 Nightwing has this
A LEADER ON ANY TEAM: Nightwing is a wild card. When you reveal your force, choose a friendly character's keyword; Nightwing possesses that keyword for the rest of the game. Nightwing can use Leadership, but only removes action tokens from characters using a team ability that Nightwing can use.

So, in a 400 point game, these two guys meet one half your named theme requirement - whatever you choose it to be - and you still have 175 points left!

So, for example, Emma Frost (M10th) and Magneto (M10th) team up with Nightwing and Gandalf for a Brotherhood of Mutants theme team! (400 points, on the nose)

Gandalf not being Modern legal will put a damper on that for most venues.

Don't forget Stan Lee. My venue is silver age and allows the comiccon pieces so I can wield nearly 300 pts of copycats.

I allow any Heroclix compatible figure to be played as Modern Age at my venue. I have yet to hear a complaint.

A venue that doesn't allow it sucks IMO.