I too have an EP2 SP Mod, which a few of you know about. You can find it here. I also have a portal map here and a Portal 2 themed TF2 map here.
I might as well share an Idea for a multiplayer mod I had. It's basically an extended role of the TF2 spy with a little more interesting gameplay/objectives. (spoiler'd to take up less space.)
Spoiler
Team 1 is the spies, or infiltrators. Their objective is one of several gamemodes: Sabotage, Recover and Assisinate. Sabotage and Assisinate are similar objectives, which involve infiltrating the base and destroying/killing the target, but the difference is that the human target will be controlled by one of the players. Recover is basically one-sided CTF.
Each spy has only one life, and minimal weapons, but can become invisible, throw smoke grenades and use a variety of other interesting gadgets. They can also find lockers and take on the uniform of a worker or guard to blend in.
They can destroy security cameras and assisinate guards as well, but the death of a guard is broadcast on the minimap, so the spies must have an escape route planned before killing someone. Smoke grenades are helpful for escaping should you get into a combat situation.
Team 2 is the guards, or defenders. Their objective is simple. Protect whatever the spies are trying to get to. They, however, can respawn and have a lot of heavier weapons such as a variety of guns, driveable vehicles (from motorcycles to tanks) and the ability to call in a helicopter gunship. This is to prevent the spies just storming the base, as they are outgunned by players and outnumbered by enemy NPCs (although, I have an imaginary achievement for completing the objective without anyone on your team cloaking once, and killing every member of the guard team at least once.)
Now of course, the guards could just go kill everyone they see and win, right? Nope. The victory is determined by points. The guards get, say 100 points for each spy killed, but lose 150 for friendly-fire. Accomplishing the objective gets the spies a large number of points, which is pretty much a guarantee of a win. However, they lose points if one of their number is killed. So, like Quidditch, if the game-end objective is completed, the team usually wins, but if the rest of the team absolutely sucked, you may still lose.
I have a few concepts for maps going on in my head, but am wondering if I should use Unreal or Source engine. I have more experience with Source, so I'm leaning towards that but I also have UDK installed and could theoretically learn, so I don't know.