A possible terminology compromise might look like this:
In general, we avoid using either Kosovo (as an adjective) or Kosovan on Wikipedia, using constructions like
- "Kosovo (as a noun) Serb" instead of "Kosovan Serb";
- "peoples of Kosovo" instead of "Kosovan peoples";
- "Kosovo's major cities" instead of "major Kosovan cities".
Furthermore, official or very commonly used constructions like "Kosovo Serb" (see above) or "Kosovo Assembly" (official name) will be used when available.
Should, however, it turn out to be entirely unavoidable to use an adjective of Kosovo in some form (for instance, because all election articles have titles of the type "Somewhereian type election, year" if a demonym exists, or for whatever other reason), Kosovan should be the preferred choice.