I think it might be kind of cool to get a Team Fortress 2 mod off the ground. I'm a bit too busy at the moment to do more than supervise and give technical support, but if anyone in the community is interested I'd be happy to poke my head in every once in a while.

Here's some ideas I've been thinking about:

In TF2, there are 9 classes: Heavy, Pyro, Spy, Scout, Medic, Sniper, Demoman, Soldier, Engineer. In DefQ, there are 7 class slots (including McGuffin). I think most of the TF2 classes could work as both defenders and enemies, here's one possible solution:

The "summoning" mechanic could be explained as the Engineer placing one-time user teleporters to summon units. Also, spells could be re-skinned as the effect of turrets.

We still have 8 classes and only 6 class slots. Scout and Spy are all about movement, so it might make sense to make those only appear as enemies. Scouts are a natural replacement for "fast" enemies, and Spies could have the phasing ability featured in NG+.

That leaves Heavy, Pyro, Medic, Sniper, Demoman, and Soldier as our core classes.

Heavy
Close-range Tank, perhaps with long-range "spray and pray" attack. Lots of HP, possibly some armor. High damage to close-range targets, AOE or "melee" attacks.

Pyro
Close-range firebug, mixture of AOE and splash projectiles. Pretty much just anything with fire. (I believe AOE attacks can hit phased enemies, so they're a natural anti-spy unit)

Medic
Easy re-skin of the healer. Replace Zeal with ubercharge, possibly throw in a few low-level attacks.

Sniper
Like the ranger, but with a focus on long-range, high-damage, single-target attacks. Possibly make dual range, and give a melee attack too.

Demoman
Long range splash-projectile attacks and AOE, possibly with armor piercing or armor breaking flavors attached to them.

Soldier
Closest thing to a "cheezeburger" unit - a medium-range shotgun for single-target "melee" attacks, rocket launcher for long-ranged splash damage.

Most of these defenders could be flipped to also be enemies fairly easily, just pick their best attack or two and re-code them as an enemy. Scouts would be really fast, and spies would have phasing. Additionally, you could make "boss" variants (ie, Zelemir or Eztli-type bosses) as well that teleport around the level.

And of course, loads and loads of robots.

Defender's Quest only lets you have 2 item slots per character, and of fixed types. I assume we would want "Hats" for armor?