In COW as it is currently, there's a number of units which are almost entirely redundant. I've noticed the following units have variants which are identical for the Chaos factions, with only a different texture, but the same model and stats:
Marauders
Marauder Axes
Marauder Axe Throwers
Mounted Marauders
Warriors of Chaos Halberds
Warriors of Chaos
Essentially, there's a Marauder of Undivided, Marauder of Khorne, Marauder of Slaanesh, Marauder of Tzeentch, and Marauder of Nurgle, each taking up a unit slot, basically like if Empire State Halberds of Middenland were a different unit than Empire State Halberds of Ostermark.
Both crunch and fluff-wise this makes little sense, since Slaanesh would never be controlling Khorne Marauders without them converting to Slaanesh worship. It only makes any sense for the aesthetic of a massive Chaos Undivided army, but even then, when you have the various faction specific units like the daemons, it's still pointless.
Currently it's 30 units there, which can be pared down to only 6, freeing up 24(!) unit slots, enough for a whole new faction, just by modifying the text files. So Marauders could keep all the variants, just making it so a Slaanesh army will look Slaaneshi, and a Khorne army will be Khorney, like so:
Files needed to be changed would be bm.mdb, edu, edb, campaign_script, and descr_strat, so it'd probably be best in the main mod, hopefully for 1.6, rather than a submod. Or if not, maybe the next Gigantus version. IMO, 24 extra slots is worth the effort, and as a bonus, the custom battle unit selections for the Chaos factions will be a lot neater, too.