You know guys, a Capture The Flag game doesn't have to include a flag. It can be a battle for some objects in the middle, like a well of souls. You know, if you capture the well, you receive a soul every turn. Or maybe some magic device the teams would have to activate, with the trick being that since the magics are different, they would have to be in possession of it for some time or start over.
Or, the teams could have some objectives akin to destroying the other team's base, or say stealing something from there. (Yeah I know, it's basically the flag at that point, but less meta.) The thing is, if you have concrete teams, you could have them be less "random being thrown in together" and more "These people have a place in the world, and have some objectives to fulfill, and have some reasons for fighting the other team." As for that object getting accidentally destroyed, there could be some fallback objectives. Harder, but there.
And another thing you can do the moment you order people into teams, is passive defenses and NPC's. Although that might be a stretch. But the funny thing is that these "NPC's" could be controlled by other people in the forum who are not playing. And I don't think that the battle should be Heaven vs Hell. These forces seem like they would exist apart from that struggle.
As for the soul mechanic, it should definitely remain, although maybe get a little more complicated. There should definitely be ways of coming back from behind. With the system as it is now, a a few lucky rolls can turn the balance from roughly equivalent to vastly overpowering. Maybe only partially award these souls to the other team, distributing the rest between the team members? That, and maybe also have some other possibilities for catching up even with less souls. Or maybe make the overall souls get awarded not for killing other people, but for possession of static objects, with the souls lost when the person dies.
One thing I'm wary of the most, is the teams intelligence. How exactly do you plan to have that work out? Moving everything to PM's might work, even if making this boring. One interesting, but probably unrealistic thing would be to put each team in their own separate forum. And then it would be a Bay12 vs SomeOtherForum. The thing about this type of game, is that knowing what your opponents are doing is very important, and the more things are kept secret, the "better" the game flows.
Almost finally, regarding respawning. Since there's probably going to be more players wanting to play this then there are players, I think that there should be a waitlist. And when a person dies, he goes to the waitlist instead of respawning. As for the one turn delay, I'm not sure this is necessary. And maybe have more than two teams?
And this seems oddly relevant if you're making teams different:
Perplexicon concept could really allow for character specialization and classes. Especially with the right arena... What I mean by that, is that you could have normal word speakers; (like right now,) but also an Engraved class, which would have more raw power but have to prepare for engagements; the Otherplane class, which would focus on deals with otherwordly deities who would give them powers (Selling your soul several times to different entities may be a bad idea); the Sorcerers, who would be using their soul (or other's souls) directly, allowing them for much greater creativity and control, but burning their own soul in the process which would lead to their rapid decay and vampiric lifestyle; mayhaps the Thieves, who would be focuses on being hidden, and casting their spells with as little notice as possible, which would lead them to being less powerful but much more maneuverable.
That's what like, 5 classes? Man, in my head, it's awesome. Piecewise must be reading this and thinking "I have just spent weeks on this, now you want to me start over?" But wait, that's not all!
With that kind of variation, maybe this system is even enough to warrant it's own world. Now all that is left, is to in addition to giving them their own class book, give each person a dozen unique words, with at least a couple useful ones. Plus, give them several words from the other classes to use. Now, you have several unique classes and playstyles, with additionally, each player having several cards hidden in his sleeve. In addition, several pages of the book could be missing. In my head, it sounds awesome.
EDIT: My brain churns on. Regarding the balance issues:
Spellcasters: What we have right now. Probably the most balanced. With proper strategy, has no advantages of disadvantages versus any other class.
Engravers: Time makes the stronger. Heavily fortified, the strongest. Probably the general strategy is creep. Getting them out of their comfort zone or doing something they haven't prepared for is key. Scouting is very useful. Besides that, don't see the obvious advantages or disadvantages.
Otherplaners (better name?): A Wild Card. Depending on what they get, can be the most unpredictable. Should probably make some crafty ways of dealing with them for each class, so they don't get too overpowered.
Sorcerers: If you can outlast them, you can beat them. It's an endurance battle. If anyone has an advantage against them, it's probably Thieves for their ability to make legs. Could possibly have an advantage over Engravers, due to the generally static nature of their work.
Thieves: The important part of this class is to make them able to disappear even if things go wrong, so there should be some skills/spells ascribed to that. Obviously, not a guarantee success of doing that every fight. Cornered, extremely weak. Should probably also have trap spells.
(And now I just feel like I'm making a TF2-style game. Oh god. Not that I don't feel that it's conceptually very well balanced, except for the maybe Otherplaners. But anyway, with all this crap added on, it makes me wonder how much the Perplexicon system would really remain integral to the experience. I mean, the point continues to be that no matter the class, you have to experiment with words, and while theoretically given the tools, you do not necessarily pocess even the majority of them. Expect maybe the general sections would have to be labeled, but that's it. But it does make me wonder how good this would be if we get those classes, spells, and skills, but without the exploration of different words part. Because done well, it would probably still be... an innovative take on TF2-like multiplayer with spells instead of guns? I dunno.)