Barkley, Stones and Shaw are 3 we should be looking at.
I'd also take a wee gamble on Wilshere if Arsenal fail to win 4th, quite similar to Ferna in style and we rely so heavily on him now. I think a lot of fitness issues would be solved by him not being rushed back by ourselves.
Lallana is good enough for the squad, but I don't see how he'd fit in the team really.
Ramsey would be a good long-term Yaya successor i reckon, provided his early season form was permanent. Goals, passing, energy. Lacks the power but there is no like for like Yaya out there. Mega expensive though and probably loyal to Arsenal for now (or just Wenger?).
There are options around, they'd just cost huge sums for their relative ability/potential.
We could also do worse than Walcott if Navas ever heads home.