Right now, nothing since buying crowns is an unneeded expense. If something came out that I REALLY wanted, I'd buy crowns from another player using gold. It would probably be a personality or weapon skin, if anything.
A template house in a plane such as: the evergloam, the shimmering sands, the coloured rooms, the sloads' mindscape. Extra furnishing slot expansions for houses (though that won't happen). A baby dragon pet (actual one, not like the illusory ones, though they wouldn't probably be lore compliant). Houseguests of my favourite NPCs.
For me to buy more crowns, they'd have to release interesting new stuff, or put interesting old stuff on the store. And there's plenty of interesting old stuff I'd still like to buy. Like, why dpes the mounts category have just one or two types for each mount, and why has it been the same for at least a year? They should put everything on there or at least cycle it more frequently.
It is kind of discouraging having to tell people all the time things like "oh yeah you can't buy this costume/hairstyle/mount right now, maybe it'll appear somewhere this year if RNGsus favours you".
"Empty Palm" Weapon Outfit. Equipping this style to your mainhand weapon will cause your character to use unarmed attack animations. Equipping it to your offhand (so your dual-wield left hander or your shield) will cause your character to use single-weapon animations.
Basic attacks with an "Empty Palm" staff would probably be reminiscent of the old Overload animation.
Basic attacks with an "Empty Palm" bow would use... various throwing animation? Because we're pretending the arrows are darts now?
It doesn't seem likely that this is feasible, but I'd definitely drop a bunch of crowns for this.