Actually, just recently Heretic made a post relating to this, I'll find it and edit it in after I post this. Sounds like tiers 5 and 6 are both planned, and his post implies they might be killer grinds (though from the Doffs 101 thread I think he expected the current setup to be a much harder grind than it turned out to be).
Way back, he also said that would be for season 6, the same time fleet advancement and possibly starbases will be, and that it wasn't a coincidence.
Edit:
http://forums.startrekonline.com/sho...d.php?t=256356
Main point is early on the first page, but there's a few related comments through that thread, just look for the gold name.
Also, I'm pretty sure the comment linking them to fleet advancement was from the old seaon 5 tribble subforums that aren't there anymore, so I'm not even sure it's possible to dig back up, but it's old enough to be less interesting than the above link.