The most important thing to me is the other player(s). If we're not vibing, it's not going to work out. I want someone who can ideally become a friend, and who wants to share canons and tidbits and throw ideas around. I love making silly lists or timelines, and going "oooh, this song reminded me of this scene" and getting that same energy in return. Make mood boards with me. Talk about our characters' star signs, which is their favourite flower, and their dreams.
There are, of course, technical things as well. But those aren't as fun to write about.
Once we're past that layer, we come to the fundamentals. I want a roleplay where we play multiple characters, and flesh out the world with numerous OCs who get to be just as developed as the main cast. Something where two specific characters don't have to be the focus all the time, and the story doesn't fall apart if there is an interesting scene we want to do without one of them. I want characters who don't have to be perfect, and get to be messy. I want both a plot and a character study. Furthermore, I love characters who struggle with themselves, and interpersonal conflicts. I want dynamics that shift
Next are the flavours. Genres aren't as important to me as themes. I like my heartache and angst, and would like a roleplay with a heavy dose of it. However, it can't be all angst, all the time. That grows dull and unpleasant. I like my levity and joy — I want to see my characters triumph as much as I want to put them in bad places. I want them to be able to overcome hardship and heal, even if they can never heal completely. Bittersweet is one of my favourite flavours. Dark themes (sometimes very much so) are a favourite of mine, and I want to be able to explore them without having to worry about upsetting someone, and better yet, finding someone who wants to explore them with me. Themes of anger, grief, immortality, regret, camaraderie, reconciliation/drifting apart, grey morality, devotion, and the passage of time (and how things change with it). I like my romance, too, particularly slow burn.
Bonus Sprinkles: Cats! As long as nothing bad is happening to them, I always like to see more feline friends in my roleplays. Violence. Awkward Affections. First times (for anything). Writing out (relevant) smut scenes. Generally tragic elements. Probably other things.
I'm going to end this by saying that I absolutely shouldn't take on any new roleplays, but if you read this and think "damn, that's literally everything I want, too" hit me up, and we can discuss and see if we're vibing.