That's the question a lot of us are asking... I think certain figures are safe like Tusken Raiders, or Ki Adi Mundi. But I'd think any of the major characters in any incarnation are fair game and it may be smart to unload now rather than wait for the bottom to fall out.
I'd say early on these previous Sideshow figures are at the most risk:
ANH:
Farmboy Luke
Princess Leia
Obi Wan Kenobi
Vader (eventually)
Stormtrooper (when it happens, we'll also see sandtroopers)
ESB
Bespin Han
Boba Fett
Vader (either ESB or ROTJ)
ROTJ
Jedi Luke
PT
ROTS Anakin
ROTS Kenobi
Darth Maul
Later on I could some others getting the treatment as well such as Yoda, The Emperor, Lando and the clones. But honestly I think those are all pretty good as-is so the need isn't there and they're fairly recent. I also think HT will stay away from Hoth and the droids for a good long while as the market is good on those, but I would not be surprised if they take a crack at them in 5 years or so.
I think figures like IG-88, Gamorrean Guards, Nik Sant, etc. are safe and will never be released by HT.