Add more units to existing factions
While many people claim more factions is what the game needs, I personally believe expanding the rosters first should be the way to go.
SoC strikes me as a game where army composition is deeper than in Heroes. With things like units generating different types of mana, being able to build multiple of the same dwelling, or just no clear-cut tier system, the game seems to encourage deviating from "one of everything" strategies and instead going for multiple stacks of the same unit.
If we look at games like Starcraft, they have only 3 factions, but each of them has a big depth, with many different strategies and army compositions to choose from.
I think this is something that would work in SoC too. More unit variety to take advantage of its unique designs.
A good starting point could be "counter units", designed to hose particular strategies, especially ones being troublesome in the meta. Or units that push underpowered strategies (like all-melee?) to viability.