- make some races naturally dislike other (diplomacy etc.)
- every race should have specific units that only that race can create, maybe even specific buildings (with certain bonuses)
- more differentiation and customization possibilities regarding look of units, buildings, cities
- different races maybe even have a different ki and behaviour (very aggressive/expanding, or defensive for themselves etc.)
- maybe even different items in shops for different races, different mercenaries that you get to recruit
just some ideas... i think strong differentiation between races (look and playwise) is one of the key-drivers for replay-value...at least it was in master of magic, together with strong differences between the magic deks....