Suggestion:
1. To have an option to overfeed a dragon so it has more HP
2. Sending the dragon requires a send button (to make the above possible)

With the max ceiling of over feeding (to prevent abuse of un-killable dragons)
Emerald - 1x of current GC limit
Ruby - 2x of current GC limit
Gold - 3x of current GC limit
Sapphire - 4x of current gc limit

As for rate of HP increase per dragon after limit, I say all dragons should be = in HP once everything is fed up to max, but their growth rate is different. (but this is personal opinion, same with the max ceiling of overfeeding)

Up to devs to spin the idea of what is good though.