In a way dragons do have a limit. They can only go up so high. As new tiers come out eventually dragons have to be stored away since you couldn’t get the next stone for said tier.
I think it would be cool to have some more variety and make it an exclusive, collectable like season dragons are. I think it could work making it like the rider lines and get the special resources in the line to level it up. Only be able to get those resources in that line like the rider branches.
Since they are a seasonal tower, make it so you can only build one and that it doesn’t give xp as you level it up. That way you don’t have to worry about over leveling by working on this tower.
Although then comes in the problem with timers. I’m not sure I would want to waste my valuable timers on a seasonal tower. It is hard enough to level up my bases towers and speed up hatching of eggs. Maybe they can include enough timers to finish off the tower in the lines. Or get it close at least like they did with the exotic rune.