This is excellent and really scratches the same itch as like, Magic Archery.
That said I've got four suggestions that might be worth considering
First off, the game is fantastic and just like everyone else I want more of it, but my first suggestion is actually to nerf the player's AOE abilities. Tail swipe clears the entire field and that means you're only ever dealing with bosses. Which is fine, but it loses some of that appeal. And Roar is really good, frankly I think its balanced on its own
Second, the enemies should scale based on time (maybe only starting after the first boss goes down so you can always prestige to reset it), both in how many are coming and in how strong they are. Being pushed into a state where you can't defend your hoard anymore and need to lay an egg is... certainly not for everyone, but it opens up the ability to add new upgrades that specifically weaken the kingdom's growth to buy you more time. Definitely cap it though.
Third, and probably the only one likely to be implemented, let us zoom out! Lung Power quickly makes the breath range go off the screen, so being able to zoom out and see the full thing would be cool. It would also let it work to actually buy time to blast incoming bosses. And, it'd let us see the full size of the cave that we're (hopefully) growing to fill eventually.
Fourth, this one's just for completionists like myself... Why not give *every* skill a max level? Even if its 200 or 500 or 1000, hitting a skill to max feels *good* and while it would take a very long time, I could absolutely see myself sitting and maxing out all my skills
Geez that was a long comment... well worth writing though this game is fire (of the dragon variety)