I think they should rearrange the weakness/resistance combinations for the majority of cards. Like making dragons weak to dragons again, and then give them a resistance based on that specific dragons typing. Making fairy resistant to dragon(I know in-game it is null) or changenit up so some are resistant to fighting. Make some fire cards resist fairy!
It'd be really interesting if cards all had semi random selection for weakness/resistance. Like 2 garchomp cards from the same set but weak to different types. But that would propbably be really expensive to pull off.
I think ist unlikely they Reprint a DDE or even print a new "Dragon Energy". ALl of the new Dragon types only Need one type of energy instead of two different ones. So a "Single Dragon Energy" wouldn`t make sense and DDE could be too OP for things like Dialga etc... just my two cents