It would be great if Weaving were the equivalent of Grand Moff Tarkin for the new timeframe and post-Imperial period. As, say, commander of a former Imperial fleet bent on striking at the New Republic to deliver a deadly blow against the new galactic government headed by Leia and others. Hugo's always had the campy dramatic chops to make for a creepy, unsettling screen villain as his turn as Agent Smith in the Matrix
If Thrawn appears he could be as a supporting, secondary role to Weaving. Maybe the two of them could organize an offensive and assault against the New Republic, goaded on by the forces of the Dark Side. Anything's possible seeing as we know next to nothing about the film, but since the EU isn't going to be followed like a map I imagine if characters like Thrawn do show up then their roles will be substantially different in the film canon.
All this said: Hugo Weaving as Grand Admiral Thrawn would be absolutely incredible.