I played Halo PC to death for a period of about a year and a half. In fact, I'd bet I've played it more than any other single game in my life. I still play it off and on, in between bouts of 2.
With that in mind, I'll say a few things about the stand-out maps that were only available in Halo PC.
Death Island: Instantly recognizable, as much of the map is a copy-and-paste of the beginning segments of the Silent Cartographer from the Halo 1 campaign. Fairly fun map, and would benefit from the vehicle changes in Halo 2. Banshees were introduced into multiplayer for Halo PC, but were nigh invincible as there was no reliable way to take them down. This led to some serious balance issues obviously, and the map lost its appeal quickly. The myriad of ways available to destroy a Banshee in Halo 2 make this map a fine candidate for reintroduction.
Timberland: Probably the most popular new map from Halo PC. Finely tuned and balanced for both medium and long range gunfights, with base designs copied directly from Sidewinder. I doubt this one will see the light of day though, which is a real shame. Or not, if you despise sneaky snipers, to whom this map caters to directly.
Danger Canyon: Another fun entry, featuring a high-road, low-road layout rife with choke points. One can either take a vehicle through the snaking valleys to confront the enemy base head-on, or go on foot through the back of your base to access a central bridge that leads directly to the same point at your opponent's base. A really fun map for both vehicle whores long-range gunfighters. Think vertical Sidewinder.
Ice Fields: The only map other than Sidewinder to feature an arctic setting. Medium-sized, fairly complicated map with may ins and outs to each base. Several one-way teleporters drop you off at a bridge overlooking the main vehicle choke point. Intense for CTF, since there are simply so many ways for the enemy to evade yo once grabbing the flag.
Gephyrophobia: Not unlike Unreal Tournament's Face, this map is supended in air with a long central walkway linking the two bases. Players not wishing to become sniper bait on their way across the bridge can use teleporters within their base to access the upper ledges that line the right and left side of the maps, giving access to several sniper rifles and Banshees. Or, you can take the Assault on Control Room-inspired lower walkway to the other side. Another wonderful CTF candidate, and would help to make Assault a little less tedious.
Infinity: Just like it suggests, this is probably the largest Halo map to date. Basically a huge figure-eight with a central choke point between the bases. Also features several tiers of long narrow bridges similar to those described in Danger Canyon. Not one of my favorite maps frankly, simply due to the fact that even 16 players is far too small for a map this size. This thing belongs in the Battlefield series.
And of course you all know the Halo 1 maps. Out of those, I'd personally like to see Prisoner and Chill Out make a return. I'd like to say that Hang 'Em High would be a nice addition, but it would befall the same fate as Coagulation. Without the original pistol, this map would be a chore.
The ones that I believe are most likely to make a return are Sidewinder (I've heard rumors of a jungle template for its return), and, unfortunately, Hang 'Em High.
If anyone has anymore questions regarding the Halo PC maps, give me a heads up and I'll be glad to answer.
