Before thinking of the really BIG stuff (Office + Photoshop clones would probably require 5000+ man years to develop), you should take care of the basic apps - a decent browser up to current standards, email, ... Nobody would want an expensive piece of hardware to run a somewhat decent Photoshop and nothing else.
Include sufficient processing power, drivers for current mass hardware, make it cheap, stir lightly and you're a step closer to thinking about 'killer apps'.
This is more ludicrous than elusive - sorry.
