I think it's really great you've decided to remake the game. My biggest concern with the first was that I felt like it played somewhat slowly. Even though the replay value was really good, I would like more depth to this sequel.
Different maps to conquer that would accompany the ages would be a great addition. Buying upgrades with leftover gold from these maps would be a must.
You've spent quite a bit of time developing different characters for each age and they all look great. You should make sure the gamer earns your time by making them have to earn these different characters. Even just starting out with basic melee characters and having to recruit new units would stand alone as a great addition to the game.
This has some strong similarities to a turret defense game, and you could take the qualities that most people like from that genre (strategic turret placement, fiscal management, and reserved upgrades) and bring them all into this game. For each map that you have, you could buy turrets there to make it a bit easier.
Replaying maps for a higher score and more gold would make the game easier for those who aren't that great at managing money or who just want to buy everything (like me). Higher difficulty levels might take away this option.
I would like to see siege units as the player advances in Age, as well as preemptive strike options or "carpet bombing" for the modern ages.
A playable hero might make the battles go by a little faster, but that wouldn't be necessary if you add enough elements to keep the player's mind occupied.
I'm really excited about seeing some gameplay. Good luck with everything!
