Gonna be Writing down suggestions I think would be cool to see one day :) :
1.Multi-Image Sets/Full Size Thumbnails: Seems to be most popular request so I don't feel the need to say more :D
2. Multiple side blogs within one account(limited to 3?): The one thing I felt made Tumblr most unique. Lets people keep their more different content separate/more organized(should they choose) and people can more easily subscribe/follow the content they want to see (be it sfw, nsfw, webcomic, tutorials, or just a place to sketch dump. Also this helps save the need for any alt accounts)
3.Tutorial Filter: Being that NG is famous for helping people grow into better artists/animators/music people, adding a "Tut" to the E,T,M,A filter I feel would be a welcome addition?
4.Photography Portal: I agree with keeping it separate from the Art Portal cos reasons, though I don't think an entire medium should be left out the site altogether? (have a blam system if need be?)
5.Dump Portal: A place to more easily find people's W.i.P and sketches? (also a gifs'n'memes tag tho not sure on copyright? wouldn't be missed either way)
6.Artist Search Bar: A Search Bar to help look through galleries more easy (the bigger ones especially).
7.Endless Scroll Option:
8.Multi-Tag Search: Using more than one tag to search at once.
Love to hear how any of these sound :)