Oh yeah... a HoMM3 scenario. I actually had to force myself not to install the game and the map editor right away, as I'm already behind schedule at my "day job".
That being said, a question to the community: which factions would you suggest for each kingdom? Obviously, Mercia would be Castle, which leaves Rampart for Varsilia (not that bad, kinda fits with all the foresty areas). Conflux or Tower would definitely suit Thuram, but what about Ilmara? Again, Tower or Castle would be about as good as it gets, but still not good enough.
Then again, it doesn't have to be strictly one faction for each player (kingdom).
As for smaller factions, I'd make Friguld a Fortress (neutral alignment, swampy jungly areas), place Dungeons where the Maze of Lamth and Sunken Citadel are, and maybe an Inferno at the Ashen Cape. The Orc Kingdom of course would be Stronghold, and the last unclaimed faction, which is Necropolis, goes to some of the major Tolassian tombs.
The plot could be about the Orc Kingdom launching a full-scale assault on Varannar, or the Undead rising to destroy humanity and establish eternal peace in death, or maybe some internal feud, or a whole new external threat.