How does each race in your game make a difference? Put another way, if any of race disappeared, how would the setting be different? How would gameplay be different?
- Remaining races might venture into unexpected niches, creating interesting twists. Elven smiths, orcish druids, gnomish sailors.
- The world might be poorer because no race or culture has filled the void left by the missing race. Perhaps metal is scarce, so armour and weapons remain primitve. Maybe medicine has not advanced, and the Heal skill is unavailable or the DCs are increased.
- Wars might be fought for reasons different than the norm, such as over unusual scarcities or rivalries.
Even though your game rules or settings supply races, give their descriptions a read over, and make tweaks so each race has a noticeable footprint in the world.
Look for ways to make the precence of each race felt during game sessions, such as roleplaying, architecture, mannerisms, fashion, conflict types and sources, art, and community quirks.