I could ramble on for days about why a hex map is way cooler than a square grid for stuff like this, honestly. There's a reason games like Panzer General, Fantasy General, Rites of War (a turn-based video game based on Warhammer 40k), Battle for Wesnoth, and so many other turn-based strategy games use hexes. A lot of strategy board games use hexes, too.

I don't know enough about Warhammer's ruleset, but I suspect it's complicated (if my experience with games like Warmachine is anything to go by), more complicated than you'd want in a forum game. I'd probably keep the lore but find a nice, simple system for throwing armies at each other.

EDIT: Though at that point you could use a lot of different settings.