Just take the ruler you get after the first one died, what is the problem here?
Like it matters if he is male or female, black or white, WTF?
Some people have preferences, and as said above Modding will likely be the solution.
I for example don't mind ethnic diversity in a unified terran government, but if the human empire ends up being the continuation of the chinese empire it's incredibly unlikely that there will be a nonchinese ruler. Similarly, if you wanted to import (effectively import it by pretending it's the same empire) a game from say EUIV where you as Britain conquered the whole world you would likely only ever see indo-europeans taking the reigns of power/authority. So for some the option to do so is an appealing one, personally I'd prefer it if a mod added in an additional phentotype for humanity specifically (so that the portraits wouldn't occupy a huge section of the mammel area) with basically a portrait select that only contains specific ethnicities, and potentially genders, it'd allow people to effectively play a matriarchal, patriarchal or egalitarian, and/or ethnocentric empire, it'd just take up like 18 portrait slots, so it'd be best done as a mod, with it's own little phenotype category.