In eroge, pretty much any range of cuckqueaning works if the writers choose to insert it. I personally prefer my eroge to have compersive cuckqueaning where there's a main girl indirectly encouraging the man to have secondary girls, but that's because that's the type I prefer in general rather than it working the best.
>I think games lend themselves to collect-a-thons more than they do to in-depth character study and seduction.
I think that games can actually do very well at in-depth character exploration but that coupling that to mechanics is a hard challenge (which is why so many "story-driven" games suffer badly for deciding to avoid that challenge entirely). I think The Last Sovereign, which you mention, is one of the best examples I've seen of cuckqueaning being explored in game language.
Cuckqueaning's an under-explored niche in general, as are erotic game mechanics, so it's not terribly surprising that mechanical exploration mostly seems to boil down to collecting belt-notches.
Throwing a half-baked idea into the pot: Marriage is often a part of kingdom management sims. If you wanted to explore the idea of a harem, that'd be a way to do it. For example, let's first assume one's spouse and concubines have mechanical effects. This might lead to events or choices like:
- The queen or main consort may suggest you take additional concubines to increase your standing… and here are some she's pre-vetted for you!
- You may find yourself having to wrestle with finding enough appropriate positions for your non-hereditary children or lose influence. Bloat your hierarchy with possibly unsuitable people, have less kids, or lose influence?
- Squabbles between your partners might need to be resolved (or could be resolved for you if one of the members is suitably skilled and given influence).
The "harem as a team behind the ruler" idea appeals on a lot of levels and has a lot of potential for further exploration, is what I'm saying. One of the things I thought was cool about AliceSoft games like Sengoku Rance or Daiteikoku was that the girls the MC has sex with often have surprising mechanical effects on the game and continue to have storylines and interactions even after their first scenes so it's a little less like a collect-a-thon.