In any game with the prefix "MMO" it's almost a given that there has to be customization options that don't concern themselves gameplay, but rather appearances. Some of my favorite features in MMOs are things like "wardrobe slots" that let you look the way you want to.
Granted, this is an RTS and the RPG elements aren't even the main focus, or even a peripheral one, but when you lack an avatar to truly set yourself apart you have to express your individuality through your units. And in a game where there are massive amounts of players (see what I did there?) it will quickly become tedious to fight with armies of clones. Even something as simple as adding tabards (re: a big ugly splotch of color on the unit texture [Re-doing the models would never happen]) and allowing players to customize their heraldry from a selection of sigils and colors would do wonders.
Now, the more wishful and fanciful side of me would want to see units with completely customizable equipment, like a modular system where a unit's role is defined by their equipment, not their troop type. That seems to be the trend in the absence of customization, but if I wanted an anti-cavalry/large beast unit to have say, pikes or spears instead of halberds (Because really, would you rather have a unit that has to rely on slashing with his weapon, or one who can plant himself right in the path of that horse and brace his pike to stop it dead in its tracks?), I could.
Obviously all of this (equipment) would correlate its strength to its appearance and level. Simple wooden pole with a simple iron tip to steel-tipped weapons with sturdy hardwood like oak instead of pine, etc. Lets avoid the theatrics of most MMOs haha.
Just my two cents on the matter, for the moment. :P