First and foremost is obviously lower the prices. However, here are some other ideas...
How about a new army type ?
A sea based army would be good - I was thinking Atlanteans...
Plenty of scope to let the imagination run riot.
The core units might comprise trident bearing sub mariners, lost souls of sailors and fishermen (net specialists), crablike men (very high armour save of course).
Special units might be mermaids (or mermen) with a tail lash attack and infatuation spell. Maybe sandmen, immune to non magical attacks - except water based ones :) How about Sirens or water nymphs.
Rare would have to be the giant poisonous octopus and the flying mantaray.
Anyway, how about that for some sea food for thought ?
Has anyone else got any ideas. If we get a few responses I'll pull them together and email GW to see what they think.