I have an idea which might not be new but haven’t seen it so far so here goes...
Once you have completed it all (including upgrading dragons) and have all the possibly bred dragons at least once. You also have to have all islands. ....
You get offered an ancient world island in dragon city. While ancient must go on there, other dragons can as well. This costs gem. And you have to get all the land and habitats.
Then when you go to ancient world it asks if you want to transfer your dragons to dragon city. If you say yes. They fly out of the portal and a nice animation of them flying and landing on island. The portal now has an empty centre and you cannot travel but get to keep it as looks nice on island.
Now... ancient dragons can breed with standard ones. Opens up new dragons bit by bit. Either they can’t fight or only other ancient dragons.
Therefore the ancient world is not forgotten but used in the main part of the game. DC makes money from gems and possibly quests and other things with dragons. Could offer skins etc that cost money. Get the breeding station type thing similar to one in ancient world but have to buy. Other buildings based on ancient world design. Could even keep platinum so that use towards things for the island or convert to gold in a converter which also needs to be bought.
And no I’m not saying paying money is good. I’m giving an incentivised reason why DC would do this.
Well there’s my idea.