We know Koeman is partial to 2 men up front and has often had a "runner" up front (ie Shane Long). I think this would really suit Mirallas who has pace and the ability to finish. I remember early into Martinez's tenure against Chelsea he had a cameo up front and looked dangerous. I was always surprised Martinez didn't use him more there as he was far more a Martinez striker than Kone ever was.
If we were to get Arnuatovic it may also allow us to move on what I consider some of the deadwood (ie Lennon, Mcgeady and Cleverley) and hopefully recoup more than we have paid.
I would have no problems if we moved Niasse and Kone on moving Mirallas and Deulofeu to be strikers as they would both be good options. In some games if Rom's not at his best, with Arnuatovic they could play as a 2/3 and cause real headaches for sides.