Treating this purely as the hypothetical question "who are the best fifteen T20 players in England" for a moment...
I reckon there are seven near enough certainties in the starting XI at the moment - Hales to open, Morgan to bat, Buttler to keep, Ali and Stokes as batting all rounders, Rashid as the attacking spinner and Willey as the left arm option. That leaves you needing a first choice opener, a three bat, an opening bowler and one more seamer for the main side, plus two back up batsmen, one seamer and one spinner.
For most people, those first four gaps might be filled by Roy, Root, Topley and either Finn or Plunkett. I don't think any of these guys have done much wrong - I'm actually coming around to Roy as he has blended more with Hales at the top of the order. Assuming the other half of my either/or gets in, we'd be looking at adding two from Bairstow, Vince, Taylor and Billings with the bat and possibly Parry or Tredwell with the ball.
Stepping away from the party line, I'd actually go for:
Roy
Hales
Pietersen
Morgan
Buttler
Stokes
Ali
Plunkett
Rashid
Willey
Topley
+ Bairstow, Billings, Ansari, Finn
Of most note here is the absence of Root - which my known status as a Yorkshireman par excellence probably makes even more shocking. Why? One, he'll break down eventually and if you do not require his offies - which in a side with six bowlers plus Roy you do not - then there is no real point in his being there. Two, he can't match the tempo the others can set, and I want us to go hard from the off. We can't match some sides for the brilliance of our batting, but we do bat incredibly deep - Plunkett, Rashid and Ali are decent, even if Willey probably can't carry his domestic hitting over to the international stage, so no need for a pacer.