As far as heroes, I think
Lysander for Sparta was a much more significant figure. He was the admiral in charge of Sparta's fleet, and a cunning tactician, who was probably instrumental in leading Sparta's navy and Sparta itself to victory. Some naval based powers might be cool, maybe leadership to ships whilst he's on board? He should probably be a decent warrior too, being Spartan and all
Pericles for Athens was the commander in chief of Athens' during the start of the war, but it seems like he was more of a political and diplomatic figure. He was an orator and was apparently incredibly intelligent - so he'd probably be good as a leadership/support type hero. He died mid-way through the war from illness, but he seems to have been a strong defensive leader.
And i can't remember if defence towers have really been discussed for Sparta, but perhaps a garrison/slaved unit type upgrade would be nice instead. You could make them drafted helots or light hoplites maybe, or peltasts if you thought a ranged thing would be better

It could be a global upgrade, or perhaps it could be a building-by-building upgrade.. The better buildings like the fortresses/barracks would get more/better units for protection than buildings like houses/farms.