If we start on the basis that a player is making their debut only in that format of the game, I'd be tempted to say Ben Stokes in Tests - no, I don't think he is anything like the sixth best batsman in England right now, nor am I in any way sure that his bowling is going to be of the required standard, but it seems pretty clear that for England to progress they need a batsman who bowls something a little more useful than Trott or Bell's rank medium pace, and Stokes is certainly the kind of player who has the tools. (Personally, I'd prefer them to play their best five bowlers and accept that Bresnan, Rashid, Broad and Swann between them would make up enough runs in 99% of matches!)
In one dayers, I could see Jason Roy getting a go. I could also see an argument for James Harris there.