When a player gets to level 80, give them the option to go prestige.
They would start over at level 1. All their special buildings, specialists, buffs and resources(in star menu
and storage)would be stored away. (To be gained back later by other means)

Move the pvp rankings button to inside the colonies section somewhere. Or delete it.
(I seriously hate that button! I click on it by accident all the time!) Replace that rankings
button for prestige level players to see what they have available in their inventory. (All that stored away stuff mentioned above.) Gray the button out for everyone else.

When the prestige player levels up, instead of gems/star coins reward, give them the option to choose from something/s from their inventory that they had before.
*Specialist, special building, buff and/or resource.

Also, you would need to add quests for them to be able to recover other items, because obviously, players that made it to level 80 have A LOT of stuff!

And the option to get access to something immediately by using gems. Less than what it is to buy it new, of course.

Give prestige players a special border to go around their avatar.

One more dumb idea... As a reward for prestiging, give those players the option to mass send explorers. Or some kind of equally AWESOME reward!