Those are some good questions. I admit almost all of my IF experience is through CoG (and video games in general in what capacity they have for this) so that’s kind of where I’m coming from as far as what my idea of an IF is in the first place, if that makes sense.
-Romance: I’m personally okay without it. It can be nice, and I’ll usually follow a romance path if one exists unless I’m just really uninterested in any of the characters (in which case I’m probably not still reading anyway), but I don’t think it’s necessary at all. You might also check this thread out if you haven’t already.
-Endings: I’d be a bit leery of set endings, but it kind of depends on what you mean. In general, I want to feel like I had an impact on the world. But that doesn’t necessarily have to be from the ending - seeing the effects of my actions throughout the narrative can still be satisfying even if the ending proper is always the same. I would generally like at least some variance, even if the “core” of the ending is the same, but I’m not sure it would be a dealbreaker.
As for sad endings…that would probably depend on where I’m at in life at the time, so I don’t think I can properly answer that. I guess my short answer is I’d lean towards not having a guaranteed sad ending because that tends to undo what I was just talking about, make it feel like in the end nothing I did mattered, and in 2020 that’s just not a feeling I want to have to deal with. But I’ve also seen plenty of powerful and interesting sad endings, and I want to say I recently saw someone talking about IF and/or CoG games with sad endings that worked great (although I forget where, I’m afraid).
-I think there’s some definite merit to the idea of playing an IF as more of a passenger than the driver. I can’t think of an example off the top of my head, but if that’s sort of what you’re describing, then I think that could be a cool idea. I’m not sure about whether I’d consider it “true” interactive fiction or not, but that label wouldn’t make or break my decision to play it if it sounded interesting otherwise.