So the first thing I would do is demote whoever this additional batsman is to #6 or #7.
Buttler, Roy, Bairstow, Morgan & Stokes are our best t20 batsmen, so they should be coming out us our top five.
If things are going well, and there's only a few overs to go and some slogging to be done, then you send Mooen out at Number 6.
So the additional batsman's main job then is if we have a batting collapse in the first ten - then they've got to come out and get us to a respectable total.
If there's anyone who's a good batsman, and an absolutely outstanding World Class fielder (I don't think Denly or Malan class as that) then you would consider them.
But otherwise - if we're really thinking short term, and trying to win the World t20 later this year......if we're in the final, our top order has failed, we're 30-4 with fifteen overs to play, and in dire need of a composed innings to get us up to a score that we might possibly defend......I think there's just one rather outstanding candidate for that role. And he wears #66.