Strangely, or maybe not, I find sports team colors make a great starting point. They all have 2 or 3 well matched colors. I've heard they are so well...
The QSRs are great to begin with. I don't think you need to stick to the actual battlebox minis per se, but you do need to stick to Warcaster/Warlock...
Does it need to be "super competitive" as in able to handle both high ARM and high DEF and be able to control/contest multiple locations on the table...