I've never had a save with Arsenal and I don't know what your budget is but looking at their squad i'd say you need a CB, a ST and a back-up GK. For CB, look at Doria (Botafogo - Costs about 11 million pounds) or Eder Balanta (River Plate - Around 7 million). Both are very popular on FM as they turn into the best in the world and they are very young. For a back-up GK I would personally recommend Timo Horn (Koln - Buyout of 3.8 million). He's very young and cheap and is more than happy to be back-up for a few years. For ST, and if you're on a latter update, Giroud is no way near as good so I would sell him for 20+million and get another striker in. If you want realism go for Alexis Sanchez who'll probably cost 13-17 million. Other wonderkids include Jese (29 million buyout and is a monster), Kevin Volland (12.75m buyout from Hoffenheim and that is a bargain), Michy Batshauyi from Standard with a 6.75m buyout, Gabriel Barbosa you can get from Santos for 2-5million or Luciano Vietto for about 5 million. If you want a 'ready made' player and have the dosh then go for Karim Benzema but he'll probably cost you 30-40 million.
If your playing on the latest transfer update then you'll also need a RB as Sagna has left and to keep in realistic you can get Debuchy (Newcastle - for 8-10 million).