I just discovered this great group of developers that are putting together a MOD that uses the idea of the original RATS, but even better. It’s called Toy Soldiers and here is the concept:

TF2: Toy Soldiers Mod, will be available in two formats. The Basic version will be in a form of a mappack, containing all the maps and you will be able to run it on a stock game server. The Complete version will include a refined mappack and a highly modified version of the game, a full installer and (hopefully) a self-updater will be available. The purpose of this mod is to go farther than any of the rat maps you’ve played before. The basic concept of this first release, will be a map for each of the rooms in a house, have them all tied in so you feel like you are just going into the next room, and not from one style map, to another. Most rat maps have a typical scale of about 1/10 or 1/12, i’m pushing this even further, due to the abilities that the orangebox engine has provided.

- Toy Soldier MOD

Check out the website here: http://toysoldiersmod.com/