Replace nofollow tags now (rel=nofollow) because it's definitely meant to specify PAID LINKS, telling search engines HOW MANY links a site is being compensated for. New keywords in rel= tags are contact, acquaintance and co-worker.
You can submit and view the approval, or acceptance, of new rel keywords at wiki.whatwg.org; some of the more interesting link rel extentions and their proposed uses being introduced are:
NOREFFERER - supposed to NOT send header information if the user clicks the link. SEARCH - specifies your site's search page. PREFETCH - what you say! Quoting w3.org, it "Specifies that the target resource should be preemptively cached". COLLEAGUE - probably the best, current bet for tagging back link partners that are related to your site.
Other relationship and familial extentions that appear to be accepted include; kin, sibling, parent (mom and dad), neighbor, date, sweetheart, crush, muse, co-resident and more; visit the official site, that list is long.
Finally, being a niche kind of site, we're trying to get a handle one rel keyword which is named TAG. The A link rel='tag' is like a referral, but in reverse. It depends on the link target to give value to the linked text, rather than traditionally vice-versa in which text of a hyperlink describes and adds value (or takes away value) to a site linked TO (like a referral). If this is a DOFOLLOW in reverse and the linked sites are good, you'll definitely want to use this as the "Bookmark" for outgoing links.