More players is never a bad thing and referral system could help here. Probably most of you know what this is, but if not... It's a reward system created to motivate players to recruit friends for the game.
Here are some examples from other games:
League of Legends
SMITE
PALADINS
World of Warcraft
HotS
Trove
So in AOEO it could work in the way when he reach X level on some civ. you get a reward or you could make account level (you could use this for a lot of other thing) and each time you level up civ. play a PvP game or Crete mission and so on. You get XP and level up. And when a friend reaches X level on his account you get reward.
Here are some examples from other games:
League of Legends
SMITE
PALADINS
World of Warcraft
HotS
Trove
So in AOEO it could work in the way when he reach X level on some civ. you get a reward or you could make account level (you could use this for a lot of other thing) and each time you level up civ. play a PvP game or Crete mission and so on. You get XP and level up. And when a friend reaches X level on his account you get reward.