Maybe not "unite all the bandits" but how about lead a band of bandits down the river band and commit some banditry while engaging in some witty banter. Then later you could all heal up with some homemade band-aids.... Wow,.. that kind of got away from me there.
I like the general idea though. Sans mods how about you pick a nice hide out (clear it of its current tenants)... Preferably a cave/fort that has a forge, worktable, enchanting table... whatever suits your needs.. Then you gather followers (4-5 should be plenty),,, You can launch raids on travellers.. (Mostly prisoner escorts I guess...

Maybe attack a village ocassionally?)
Bandits are a loose knit bunch anyway... Uniting them all would probably lead to a 3rd faction in the war.