I think this season has shown us that we have an excellent starting eleven, but that the collective can quickly fall apart if certain players are missing. Proper back up for Kane is the obvious one. Would also like at least one quality central midfielder. When Dier/Dembele are not playing we look very weak. Someone strong who can dictate play would be great.
I know Bentaleb is very young, but Poch seems to have given up on him a bit. Might be an idea to loan him out next season, to see if he can get back on track. Mason is full of passion but not so hot when it comes to ability. Carroll should only play AM I think, as he doesn't have the physical presence to control the central midfield. Selling either one wouldn't break my heart. Ok, Mason a little bit..
There was talk last summer about Axel Witsel. Haven't seen him play much, but going on reputation I think that's the type of player that could improve us.