I think people will be able to better answer your question if you elaborate on what kind of game it is. I mean, the vision that I get from what you described is more than likely very different than what you actually had in mind. I'm imagining something akin to those old zombie flash games that used to be super popular, where WASD was your movement and you'd use the mouse to aim where you wanted to shoot. Except well, the obvious different is that there will more than likely be 'interesting' terrain, which is the topic of your conversation, and instead of zombies, other players to kill.
Some questions you might want to ask first though...
- Are the players 'equal' at the beginning of the game? If that's not the case, is it something they can change during the match, or a permanent setting depending on the match? (IE, do they select a couple of different 'classes' that determine their base stats, or can they choose from one of five starting weapons, that kind of thing.)
- What kind of power-ups do you want in the game? Are there only weapons to be collected, or are there other things like increased speed, extra health?
- How is your hit system? One shot and you're dead, or is it more akin to having lots of health?
- What are the game modes? This is probably the most important question in determining what the maps should look like as a whole.
I'm just kind of speaking my mind here, but these are probably some things that you want to keep in mind for a level design of a top down game.
- You want to focus the action in a couple of key places, much like first person shooters. I know there are heat maps online (I know Bungie did this) and since you're experienced with Halo, you might be able to determine 'why' players were dying in those places so much.
- Power ups are central to the movement of the players. They always want to be moving (Unless there is a sight mechanic in the game where you can't see around corners / in dark spots) to another location to become stronger, find another man to kill, or otherwise run away from a bad fight. Power ups are where they're going to go.
- Choke-points. Sometimes funneling players into tight corridors is the way to go. Huge open rooms are only so fun and strategic. That narrow hallway which contains the rocket launcher is not only appealing, but dangerous as well. You can bet lots of players are going to die near there.
- Bases should be defend-able. Power ups or weapons should be aplenty for players who are killed and have to respawn, and perhaps even accessible only to the team who lives out of that base. (Force walls that only let them through but bullets can pass, something akin to that?
- With a top down game implementing Z is, in my opinion, not all that intuitive unless everything takes place in doors and there's no way to interact with another level (besides moving between them), mainly because of graphical concerns. You can't see both a man up on a ledge and the guy right below him. Pitfalls I don't think are necessarily appropriate, but depending on your game, other kinds of traps certainly are!
- Environmental Hazards. Are they a part of your game? Saw Blades, crusher traps, shifting walls, 'random' force fields that can be activated or that pop up? These can play a huge role in where players go, where they want to go, and where they can go without being turned inside out.