This has been a rumor rolling around in screeshot form for a month or two now, but over on Facebook it looks like Mark Acheson has opened up and let everyone know that he will be returning to play Lord Tirek some time in season 8. The original screenshot noted that it would take part in the finale, but we still can't find the direct source for that one. Regardless, his Facebook page has quite a bit of season 8 Tirek hype on it, so I think we can call this one confirmed.
Screencap of the most recent announcement below.
(Original UNCONFIRMED Screenshot)
Thanks to Gb and Adam for the heads up.