Arsenal could use a quicker left-side full back, who can develop a tighter playing relationship with the left-sided midfield/winger.
Right now Ozil-Sanchez and Walcott-Bellerin are strong partnerships. But Monreal isn't quite fast enough to get back if he is further forward and so he plays a conservative game, leaving Oxlade-Chamberlain and Iwobi slightly less supported than Walcott on the other side.
I guess a left-footed midfielder-winger would also be a good signing. But the squad is very strong at the moment, so there are not many positions that can be obviously improved, except by buying a superstar. And that isn't how Arsenal do things.