I don't really see the need for Halfpenny. He adds less than most back 3s in general play but is an excellent kicker. So is Sexton, so is Farrell, so he's not so essential.
At present for the backs in general i think Hogg, Murray, Sexton, Liam Williams, and Farrell have to go in my opinion, unless there is a severe drop in form or injury. Last year i would have backed Youngs to be pushing CM for the starting berth but he hasn't been great for England in the last 2 matches. There are a lot of very good players out of form at present e.g. Youngs, North so i think the fight for places is quite open. Few of the English backs have looked dominant enough to be an automatic pick, but they have the potential for several players to step up and make a claim. Similarly if Wales win their remaining matches and their backs do a bit more then several players could come through- there is lots to play for which is exciting.
Some of the picks will depend on what style of play Gatland wants to use, e.g. of the England backs Teo would fit better with Warrenball than JJ (not to say either of them are favourites to get a spot on the plane), and i would wonder if he'd prefer a fly-half like Biggar or one like Ford if both were on form?