some very very interesting responses here. Many are not Arsenal quality.
Willian is a great option, usually can be had for around 15 mill (US). He wil need to be retrained to the AML position though.
Assuming you want an IF, you have Ox, who should be no less than the 1b starter, he develops into one of the worlds best if you do it right. Nico Gaitan is an option (he is hurt a lot though). Iker Muniain is another obvious option at AML.
Douglas Costa should be played on the right, but if you want him he is generally easy to unsettle, then Shakhtar will offer him for between 10-20 mill US.