While playing for Manchester United, Fellaini’s international future would be better served by staying at Everton, under Roberto Martinez.
David Moyes, Fellaini’s former coach at Everton and the new manager at Man U, preferred to play the afro’d Belgian high up the pitch and use his size and power to attack and score goals; whereas Martinez sees him playing in a deeper, defensive role.
Since Carrick has the deeper role locked down at Manchester United, there’s no reason to believe that Fellaini wouldn’t be pushed higher again under Moyes.
But at the international level, if Fellaini is playing in an attacking role, he won’t get a sniff in the Belgian team because of how stacked they are in the attack. And if he’s not playing regularly in a deeper role, the Belgian coach is unlikely to trust him in that role.
If he stays at Everton and makes that holding midfield role his own, he becomes a near certainty to be a starter on a squad that is officially a dark horse contender for the next World Cup. If not, he will spend it on the substitutes bench.