I think with the updates, and new servers is the opportunity to create a new, higher tier of troops, buildings and researches, that are only available later in the game, but keep people playing to progress.

Troops that carry more resources, and maybe even high level defensive units.

Buildings that can greatly increase storage capacity, maybe without increasing safe amount. Maybe even a 3rd tier primary resource (hydro) mine.

Research that decreases salary cost, decreases research time, or offers a recycling percentage of lost units in attacks/defends.

Just a few ideas...

Perhaps even temporarily available units. ..