Well the most popular maps the last 4 years are the ones RED's dominate in often(Cairo pyramid, dunger, warebloom(s), EP, Military Area etc.). The trend likely being when you die first as a RED in those maps your prob only waiting 2-3 minutes compared to a large open map waiting 3-5 minutes, when REDs do well that means more rounds and less time dead and idling which is the time players leave at a large %. Also the fact that when RED is doing well 20 players are "feeling themselves" positively compared to 3 BLUE players being down while getting wrecked compared to vise versa.
But you don't want too many of the same type of maps since that style of map can grow stale, and often these maps appease the newer players more than the regulars. You should try to have a few maps good for hales and a few maps good for RED's so the community can decide what it wants to vote and play on. I believe your main objective overall is gathering high quality maps with good game design then afterwards worrying about the balance and statistics.
Personally you should ask around about Urban works and I think it is frowned on a bit more than you think
@Meowcenary ......at least by regulars.