Well Dembele is excellent defensively but is at his best when he runs at players with the ball. I would be trying to convince him to do this far more then he seems to do with Parker by his side.
My three mid's would be:
Creative - Defensive - b2b
Dembele would play the b2b role, he wouldn't be a classic b2b, but i think it's the type of role he would excel in.
Picking up the ball, spinning past his marker and running at the defence, whilst Bale and Striker attempt to run the channels or break the back line and on the other side a player keeps the width and keeps himself available for the easy pass.
Added to the fact that you have three in the middle, Sandro and the creative would be free to wander off if they feel it is needed. Sandro on one of his powerful runs (though he needs to work on releasing the ball at the end of them) or the creative could move forwards and combine with the front three. It would just be a far more fluid formation in my book. Any of the middle three could go forward, whilst the third would be able to do a decent job defensively.
Granted against the harder teams i would be more likely to tell Sandro to sit back more though.
To top it all off you'd have Walker overlapping Bale keeping the width on his side and giving the left back even more to think about.
This is just how i would play it though.