We not only need somebody to start over Arteta, we also need a proper Ramsey backup. Wilshere should be moved up front, don't like him in CM and I think that his best attributes make him most useful in the final third. Get a steady veteran willing to sit on the bench and provide some of the stability that Ramsey gives us at his best. Marchisio for example is a good profile but he's too good to sit on the bench for us, so a slightly weaker version of him. Schneiderlin/Ramsey/Arteta/Marchisio-lite is a proper midfield unit. We need stability, control, and physicality.
Then get a defender (I'd argue anywhere along the backline works, the only definite starter is Kos, and Chambers and Monreal can play multiple positions) and try to upgrade GK because Szczesny has had a shocking 2014 and Ospina probably isn't good enough.
GK - new GK, Szczesny, whoever
DF - Gibbs, Monreal, Koscielny, new DF, Mertesacker, Chambers, Debuchy, Bellerin
CM - Schneiderlin, Ramsey, Arteta, vet B2B
AM - Özil, Wilshere, Chamberlain, Cazorla
FW - Alexis, Welbeck, Giroud, Walcott