Personally, I'd expand your list and get the old 1987 Headmaster crew - Chromedome, Hardhead, Brainstorm, Highbrow, and Weirdwolf/Wolfwire, Mindwipe, and Skullsmasher/cruncher. Having the full set of reimagined Headmasters would be pretty cool IMO.

If you're only after 2, though, Sixshot would be a good standalone figure. Though of course, Trypticon is supposed to be released under the TR line too...
For an Autobot, it sort of depends. If you're a Headmasters (as in the Japanese TV series) fan, Chromedome. For a good standalone figure who could work well with Sixshot, maybe Powermaster Optimus Prime. For a good standalone figure who has good feedback, I'd go for Wheelie.