Koeman would be a decent replacement. I just love the way his teams play fearless football and are able to get wins away to big clubs. He has lost about 6/7 keys players over the last 2 years yet he still managed to produce good football and god performances, this season they are on a record points tally I think and that is mainly down to him.
Our board rely on Wenger too much though and will continue to back him until the day he dies. They don't care about titles and winning, that's obvious by now. Soon as he gets them their 4th place trophy they offer him a new deal. If wenger really cared about the club like he says then he should announce right now that he will leave in the summer of 2017 and give the club time to find a decent replacement. Apart from those managers already at big(ger) clubs, I cannot see anybody turning down the job.