I'd like to see the Table of Contents floating above content with a little bit of translucency and fixed on the right side of the page (with an overflow to accommodate large TOCs, ensuring the position is beneath the header and not high enough to extend to the footer). (Of course, one should still be able to hide the TOC.)
- Table of Contents can be very long, requiring a lot of scrolling by the user if they just wish to start reading (without going through redundant headings).
- It can be handy to have scrolled down anywhere and still be able to quickly jump back to another section elsewhere within the article.
- It can be nice to get a bird's eye view of the content one is in the midst of reading and an awareness of the position of one's current position within the hierarchy.
I've added some CSS which works for me in all 4 default-available MW skins: https://meta.wikimedia.org/wiki/Help:User_style/floating_Table_of_contents .