You can already access the SVN, test things out, and post your opinions in the resource and mapping forums. We're looking for more discussion in the monster resource forum, because a lot of work has to be done in there.

That would totally kill the uniqueness of the sons of D'Sparil. I imagine this thing now more like a special Council of few members instead of countless of them being introduced at the end of E1 and then appearing more often in the following episodes.

In my opinion, if each Son was unique they'd feel kind of underwhelming, since they're just slight sprite edits and recolors.

We could reach a balance - 3 at the end of Episode 1 (fire lightning and ice), throughout episode 2 we have a water and a metal miniboss... episode 5 we have a time and a void. Each type only has to appear once hehe!!!!

Personally, I'd be fine with using the weaker ones more than once, as long as they're used very rarely, so it does still give that Council Member feel Tormentor mentioned.

True true, or the idea of having each type be exclusive... definitely a council. We could have a separate 'magma' version for Ep3 or something so he's still a separate council member from fire....

What about having sort of Sons and one Head of the "theme" for each episode?

For example the more you proceed to the end map of one episode, the more Sons of X you will encounter. At the very end, expect a powerful master of the sons as boss battle.

And at the very end of the game, 4 masters against you in an epic boss battle in the void

Hmm, I dunno. I imagined they would be an elite group, D'Sparil's inner circle, each with unique powers and personalities. They wouldn't be mindless minions, but tyrannic lords ruling over land, planets or even planes of existence in D'Sparil's name. Each would be a general of D'Sparil's army in each episode, and ultimately the final boss. Now that D'Sparil is gone they may have become rival warlords eyeing up each others dominions, only to have their plans disrupted by Corvus wandering into their area. Perhaps they could unite against Corvus at the end? Maybe they combine their powers to summon some sort of demonic abomination at the end for the final battle, something even more foreboding than the Serpent Riders themselves?

Here's some thoughts on their character:

Son of Fire: Proud, prone to fits of rage but respects strength and courage.
Son of Water: Cold, calculating. Not easily provoked.
Son of Metal: Ambitious, always trying to expand his realm. Not very subtle in his tactics, but hard-hitting.
Son of Chaos/Void: Eccentric, prone to madness and unpredictable. Master of the most arcane magic.

Thesd are pretty cliché I know but would be nice to individualise them IMO.

Yep, Toenial actually summed it up in a nutshell! That would be the perfect solution in my eyes.

Alright, if we make each one unique (which I still don't agree with, for the record ), then let's go with CodeRed's idea of using new varieties as minibosses, as well. It would involve making some new types, but I can handle it.

Also, I've started some work on a Heretic/Hexen campaign setting with edited D&D rules with a few friends. I'm basing it more on the modding community as much as the original game.