They're not all around max level zones... LoL
In CoV the first 'mix' zone you can go to with villains and heroes is.. The Pocket D. What's that? Well it's a noncombat dance club zone. Fun place to just go look pretty and run into folks who use the zone as a warp.
The next place is, Bloody Bay (levels 15-25, you'll get capped to 25 like everyone else, but you'll only have the abilities of your level. If you're over 25, they black out and can't be used. But your slots are always in affect, so if you're level 25 and go in there, you'll probably still rather weak against some bored level 40 or 50 player who's maxed slotted. That'll make more sense later on.) Bloody Bay is located in Cap au Diable for us Villains. It's a big red dot on the map, near the airport in the zone. There's some NPCs in there worth some great xp up until 25 and the quests there seemed to me to offer up more xp than the storyline missions elsewhere for the level. Players can stop you, but on Champion where I play if you see someone doing missions, you just leave them alone. No need to gank someone more worried about where a place is than if someone is following them.
20 to 30 offers up another Villain vs Hero zone which has a twist to it. In Siren's Call, entrance located in Sharkhead Isle, you can aid in map battles between Arachnos and Longbow NPC groups to sway influance over the map. This influance helps increase the variety of merchandise on a shop NPC near the entrance. Also you get random 'bounties' which show up on the map. These bounties are other players that if you kill them you gain a certain number of bounty points which can be used to purchase special inspirations and I believe if you gain enough dual and single orgin enhancements. Basically for free just for being an active PvPer.
Don't forget Mayhem Missions. You don't really go into Paragon City with active hero players when doing these, but you do get a pretty snazzy and fun mission map where most everything can be destroyed while you go to rob a bank. Which, you'll always get to meet a 'protector' named hero who'll try and foil your plans. Fun stuff, really brought some life back into the game.
If you guys have any questions you can contact me without joining up on the Champion server. @Stogie Sam is my global. I can find info pretty quick on the game and I never mind helping out where I can.