I'd like to see changes to training.
First, it takes way too long. We have so many dragons now, that training them is a challenge. I try to constantly keep one in the training center, but I can barely keep up with training the Heroics and an occasional Legendary. They could handle this by halving all training times, or better by having a second training center or even a third training center. I don't know how that would hurt the game in any way. Charge me gems for it if you want to make money.
Second, I think we should be allowed to re-train. If SP is changing the way the rules work, by changing the combat matrix and making the Ancient World elements more useful, we should be able to correct our past decisions.
Third, the interface needs some work. It lists all the dragons in some order but I'm not sure what. Some of the ones that have already been trained 4 times are grayed out, and some are not grayed out, but when I click on them it tells me they can't be trained anymore. Then it hops me back to the top of the list. I can search the dragon type but not by name, stars, rarity, elements, etc. so I have to go out of the training center to the Dragonbook, figure out which dragon should be next on my list, and then go back into it and type that name in the search. This system was fine when I had 30 dragons, but now it could use a facelift.