My first thought was actually Doom. Take our imps, enlarge them, paint them brown, and give them fireballs to throw and you would be pretty close to the Doom ones. The overlord guys don't have horns? Meh, still a decent image though, just add a smattering of random undead.
I am not confident in the "tough Lich" idea. It seems like a lot of actions would be spent with more modification than improvement to take its focus away from range and magic... Perhaps some sort of necromantic undead strength spell, leveraging necromantic undead control for a form of undead necromancer self-telekinesis? Then it could carry a
riot shield Shadecrystal tower shield in one hand and a ballista in the other? And give it an extra pair of tyrannosaurus rex-style arms to magic with?
Normally I would suggest that a short blade like a dagger can be very effective in close combat. I mean, The Roman Legions did well with The Gladius and that is not huge. But imps are probably going to be fightingthings bigger than themselves, so spears/halbers/
voulge-voulge-bill-bill-hook-fauchard-hook-fauchard-awl-pike, javelins and axes all seem good, and hopefully we can use fireballs instead of polearms and javelins.
We really ought to try to have a tactic too, blank slots in your army design tend to be bad... On that note, I expect that our first few turns will likely be filling in gaps rather than specifically opposing our adversaries. Are there any thoughts on what we need in order to fill in all the blanks? Typically it would be fronte-line, backlline, heavy artillery, organisation, logistic, reserve/cavalry? All requiring a unit and a weapon, although there can be overlap? Meh, imma just speculating at this point.