Barkley and Coleman on the flanks sounds revolting
Barkley has actually been showing some promise tucking in from wide positions. He's an absolute motherfucker to defend in the half-spaces and it creates a bit more confusion for defenses to have that player moving outside in (does the fullback follow him? does a midfielder pick him up?) than inside out (where a midfielder can easily track him). I think it can be defensively viable too since the sideline is there to help him out (we already know Ross isn't the best at defending in the centre of the pitch). It's an experiment worth seeing through and I hope Koeman sees it that way. The 4-2-2-2 is a workable tactic if you have the right midfield two. Leicester played a version of this last year and Leipzig have using a similar tactic to great effect this season.
On the other hand, w/r/t Coleman-as-winger... I agree with the "revolting" assessment. (Might work for one game, though.)