Since OSRS is on steam now, I was thinking that there should be a category for getting every steam achievement. Every achievement is meant to be reasonably completed in f2p, so it wouldn't cost anything to start a new run, and all the achievements are relatively simple things, and I feel that finding the best way to complete them, along with coming up with a balance to quickly get the 100 total skill achievement done optimally would be an interesting challenge

Though I like the idea of more new F2P categories, Steam Achievements does include Dragon Slayer (https://oldschool.runescape.wiki/w/Steam_Achievements). That would be quite time consuming, and there's still only one run on the boards for it right now. These would include that and more.

But eh, always like to see new F2P content/categories if possible.

Steam achievements aren't rerunnable, though without creating a whole new Steam account which would only be the off-putting thing? Not sure how quick it is to create a new Steam account compared to making a new OSRS account.

The category could just be to complete those tasks without actually getting them on steam, so you would only have to make a new OSRS account like with any other category. Personally for me the biggest concern is what AnimeFanX11 pointed out: Steam Achievements is pretty much Dragon Slayer with extra steps. One way to make it different would be to make it a "non-ironman IGT" category. Currently the only IGT category is P2P Mithril Gloves.