Yeah, the vermin lord is kindof the Balrog for the skaven .. Id think have it as a unit which costs as much to make as a small wonder (maybe 300 shields?) or whatnot.
Maybe a city improvment (non wonder) which summons one every 3423626 turns or something, to prevent large numbers being built with pay-to-hurry govt types..
i digress.
For a skaven civ to be really viable, we would need a few basic units;
the clanrat (done)
skavenslave (could use the settler if it had an attack anim)
stormvermin (think elite rat troops, halberds, roman-esque armour)
giant rat and/or rat swarms
rat ogre (done, i think?)
the warplightning cannon (done)
big head-taker like the vermin lord or possibly the old skaven doomwheel
http://www.geocities.com/warpchylde/Skaven_Doomwheel__side.JPG
http://cyber.aranha.fr/spip/IMG/jpg/Skaven_Doomwheel.jpg
some units which would be "nice" are a skaven assassin (think rat ninja)
plauge monks (robed rat monks)
weapon team like the ratling-gun or warpfire thrower
jezzails (skaven snipers with long range warpstone rifles) - good "stealth attack" unit
warlock engineer (techno wizard)
gray seer (grey furred skaven wizard in a robe)
and lastly,
some form of appropriate boat .. ideas?