The Athenians are still going to have the same problem that plagued them, and that is they treated the other members of the Delian League like subjects, not partners. This will always encourage revolt, and coalitions against them that powers outside the league would try to exploit. I'm not sure how you would get the Athenian Empire to reform significantly. Even the modest reforms in the Athenian Confederation that emerged in the mid-late 4th century were only really possible because Athens had learned from defeat that giving the other members of the league virtually no say would end badly. And yet they didn't learn enough, and the Athenian Confederacy disintegrated.
So let's assume you go with the victory at Mantinea. Spartan power is crushed, the Athenians encourage a Helot revolt, and the Spartans sue for peace, severely weakened by the war in a way they would not be until Leuctra IOTL. Athens still has Thebes and the Boeotian League to contend with egging on revolts in their empire and the Persians likely providing coin to encourage this as well, since any strong Greek state is not really in their interests. Unlike Sparta however, Athens certainly has the military, economic, and naval capabilities to maintain their hegemony, or to at least put up a significantly stronger resistance than the Spartans were ever capable of. So you won't see Athenian hegemony merely result in bowing at the feet of the Persian king to prop them up like the Spartans were forced to do. Athenian naval supremacy would still be intact, and in any case, on its own Thebes is not yet capable of posing any significant threat to the Athenians in a way they were when backed up by the Peloponnesian League.
Citizen armies will still decline, to be replaced with more effective professional mercenaries, who provide more campaigning flexibility (Mercenaries have no farms to return to, so it opens up more of the year for potential campaigning, for example). You're still likely to see significant threats to emerge from Thessaly like the Thessalian tyrants (think, Jason of Pherae) of OTL, though I imagine the Athenians would be much better positioned to intervene in Thessalian affairs than the Spartans were. The Thebans will have a lot of influence in this region like IOTL.