When new max level 80 was introduced I heard a lot of people saying "why should I bother?".
Imagine you could obtain a special "epic" building after hitting lvl80. Something truly amazing.. underwater pyramid, little side island with a temple, or something similar.
Now there are two things/purposes I can thing of:
1. esthetical: you will still gaining experience after top level and after each experience milestone, building will receive another graphical 'upgrade'. Like a statues lining the road to building etc.. You simply level up your epic building.
2. functional: long cooldown activation ability.. And what would be truly epic, worth grinding the experience? Throwing all your island to star menu.. all buildings with their upgrades! Allowing you redesign your island to your desires.
more recipes could be also introduced, but to be honest: more resources, more troops, more buffs its enough to lure people into grinding.