Obviously skill capes have been talked about quite a bit and I hope that is on your list already!

Given that you've said cosmetic- It'd be cool to have more non-combat clothes options that don't suck. There are plenty of cool robe colors and such, but beyond that there's like... pink skirts. Like 2 colors of boots. Gloves. That's about it. It'd be interesting to have some somewhat common just regular clothes items.

A few ideas:
Make rings able to be equipped. Why the hell do they even exist other than for high alching. Just have them go in the same spot as gaunts/gloves.
Make ammys distinguishable when equipped- It'd be cool to know who has str ammys, who has dragonstone ammys, etc.
For that matter, change the sprites so different weapon types are visible when equipped. Original RSC was so fucking lazy, haha. How hard could it be to make daggers a little bit shorter, 2hs a little bit thicker, not have battle axes and hatchets look exactly the fucking same when equipped? lmao
New, fancier robe types wouldn't be amiss, rainbow of colors and such.
Purely cosmetic hats? Tophats, bowler hats, tricorn hats, stupid medieval wigs (barrister wigs and such).

New holiday rares so people don't cry about their economy getting wrecked every time a new santa hat appears. Maybe the complete santa outfit set. Wearable pumpkin head for halloween. Leap Day William costume for leap day this year.

Less-than-cosmetic ideas:
I'm sure everyone will want completed dragon armor set.
Maybe introduce wands as another magic item?
More armor types in general, some at same levels as rune/dragon, etc. but with a different look. Or trimmed armor, then we could get armor trimmers on our server

And here's my most OP suggestion. If you ignore the rest of the post please implement this one:
An underwear item that removes all player's clothes so they're naked except for underwear. -10 armor bonus. A whole new type of staking - NAKED STAKING. Play at a defensive disadvantage and look like an idiot. It will be a hit, I swear. Mods could have lvl 10 naked mud wrestling competitions for big prizes. Ideally underwear would be tighty whiteys.

Full dragon gear, skill capes, some new looks to pick from when at the make over mage(like a slayer(the band) t shirt ). The ability to dye rune large feathers to help tell teams apart from each other. Maybe a max skill total cape. Maybe a graphic update so 2h handers look different when wielded. Maybe some kind of hoods that came be warn.

Honestly you should just release new colors of the same rare items we already have.

+1 to more initial character customization options. Or make them only available at make-over mage for the fee.

Skill capes
quest cape
Ability to use a dye on a helmet and change the feather color
Dragon armor - with same stats as rune

Skill capes
New rares