It depends on the genre... if we are talking non harem then I think these are things I'd like to see....
a) characters that (more or less) behave like actual people, no instant attraction between people who hump each other 5 minutes after meeting for the first time, instant attraction itself is fine of course, it happens in real life all the time as well, but things still need a proper build-up
b) a compelling story that is not separate from the romance, but where the two are interwoven
c) not every female should be a bi-curious group sex fanatic willing to share, at least one main LI should have the option to be faithful to the MC (and vice versa) and not be ok with sharing or cheating - IF said LI gets a proper amount of content, which directly leads to the next point
d) focus only on a few LIs, flesh out their characters and arcs instead of adding more and more and more women to the game as it progresses to either stall things or to pad the game so you can keep it going longer than it needs to be. Avoid the common trope of making one or several LIs unavailable for a chapter or even more (because they need to leave town or have to think things over, are angry etc) so the player is virtually forced to romance other potential LIs. It may be easier to just add new girls, but it's more rewarding and meaningful if there are only few LIs for whom the relationship deepens and develops over time
e) sandboxes are usually a mess and very tedious, if it has to have some kind of free roam always make it VERY obvious when and where you need to be, but then it might as well be linear anyway, so sandboxes usually add nothing of worth in my opinion
f) avoid grind at all costs, it's just boring, having to repeat the same event with a certain LI just to raise points is tedious
g) and please don't push the sliders for breasts, hips or waist up to 11. The female characters should resemble normal human beings, same goes for the MC of course, horse appendages are maybe the silliest trope I have ever seen in VNs
