I'd go nuts over Thousand Sons, largely because Deathguard can be done with the current Codex as a pretty strong list, so all you'd get is a more fluffy list. But Thousand Sons need actual stats/rules help, so a supplement for them would both add a fluffy list and, hopefully, make the units a whole lot better. Plague Marines squads don't really need help as they are strong enough now, but Thousand Sons have some big problems.