I don't see the point of adding in some stock maps to the rotation, frankly. It's not like there aren't tons of servers out there already playing them constantly. Besides, custom maps and stock maps attract different crowds. Sure, having a stock map playing might lure in one of the casual players who is defiantly loyal to one or a few maps, but as soon as it switches over to a map he's never heard of or played on before, he'll just go to one of the other thousand stock maps servers.
I can't exactly picture it actually harming the server's attendance rate a great deal, but not much could at this point.
My best idea would be to simply refine the map list to fewer, higher quality maps. Let people build a rapport with a handful of top-notch maps rather than constantly cycling from random map to random map. It's a bit daunting to have a mapvote come up and there's pages and pages of map names you've never seen before. It was great when we just had the six CAMP2 maps on there. We just kept them around a bit too long.
But the problem is that tastes vary widely. Whereas some (myself included) prefer the deeper, more complex custom maps (Aura, Tunisia, Strasbourg), others prefer the Orange-like arcade maps (smallhill, Crash). Personally, when I just want to fight and not worry about tactics, I just head to the Orange server. That's what it's there for.