I think you're going to get a lot of people saying, "First off - make the game
you are interested in making." I'd also add the bonus of, "Since many of us here are devs or aspiring devs, we'll probably be skewed in comparison to those who are purely players." But I suppose if you just take our posts with a grain of salt, there's no harm in it.
(I've been catching up on the forums and what I've missed over the last several years when my attendance was spotty - so I've definitely seen similar threads, but I don't know if any are particularly recent.)
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My most general answer is, "
I want to see projects the creators themselves are interested in." If it feels like soulless pandering to a demographic, I lose interest very quickly, assuming I didn't feel it from the description of the game itself and just avoided playing it.
To answer more specifically some of your questions....
I usually don't play fan games. Not of "current" properties, anyway. Public domain properties (fairytales, classic books, myths, legends; even far-history using historical figures), I enjoy a lot. I actually spend a lot of my creative time playing with adaptations of fairytales/folktales - I've always enjoyed that sort of thing. I suppose the reason why I avoid "current" properties is because there's always a possibility that the intellectual property holder might release new work(s) and I don't want to start confusing "canon" and "fanon". I like there being a definite "the canon is complete - so now you can work around canon to tell your plausibly-happened stories or jump-start your alt-universe stories". It's probably the biggest reason why I don't do more than the occasional piece of fanart and roleplay in universes with original characters anymore. (I used to write a LOT of fanfic.) Just a personal preference!
I do love original works too, of course! I enjoy exploring new worlds. If I had a choice between an interesting adaptation or an original world... I'd probably play both. (If I had to choose, it'd come down to personal merits of each game, I suppose.)
As far as MC(s) go, I guess I typically prefer an actual "character" as opposed to a "custom MC". But there are some more sim-type games that are more fun with custom MCs, so I guess it's down to what sort of game it actually is. If it's a game that lets you customize your story a lot, I prefer a custom character. If the game has a more "linear" story, I prefer a proper character with a personality of his/her own. I'm not really a big fan of the blank-slate MC.
With art styles - I'm open to just about anything, provided it has care put behind it. I do prefer less amateur art, but a strong and interesting style and some polish on the graphics can make up for lack of experience. I suppose I do lean more towards anime/manga styles (particularly various shoujo styles), but it's not an overwhelming preference and depends on the type of game it is.
In terms of theme/genre... I strongly lean otome or BL romance in fantasy, modern, or historical/alt-historical worlds, but they're not the only things I'm interested in playing. I love adventure, mysteries, and supernatural elements - and I'd love to see an intense political thriller (kings and queens playing world chess sort of thing - not dumb politician pantomime), but I don't know that many people would tackle that sort of thing!
Making it clear because I feel more comfortable if I do - these are just my general preferences. If a sci-fi fan game with blank-slate lesbians who aren't courting or maintaining a relationship came along and it seemed interesting, I'd give it a go too. xD