The glass Dalek would be interesting-there's plenty of 'clear' figure variants out there (although none that I'm aware of for Who). Although the disembodied head/Dalek mutant might be difficult for various reasons.
I'm suprised we don't have any Dalek Emperors. We've got plenty of Supremes.
There's three variants of the Emperor seen in the series:
-The big one from Evil of the Daleks. But you'd probably have to do the entire set around him too.
-The small Davros one from Rememberance of the Daleks. Could have a lid that can open to reveal Davros.
-Of course the big new series one. Anyone know how well the deluxe figures of the Reaper/Face of Boe/Racnoss have sold? This would have to be even larger possibly.
I also don't think there's a Revelation of the Daleks Dalek out there either (They look the same color as the Imperials but are different cosmetically). Maybe a future box set, with Terry Molloy style Davros, the Davros 'great healer' head, that Dalek and the glass Dalek is possible.