There is no set order to do things, I suggest you get what you need.
If you need a source of light, and don't have it yet, it means you still need to find that, if you need a way to swim against strong currents, then you'll need to go and find that first. You get the idea.
For what it is worth, there are a total of 8 forms, and you won't get form #8 until the endgame.
As for the side quests/mini bosses, just go to wherever you haven't explored the map yet.
As for the secrets, you could miss them even if you knew where you where at.
As for the treasures, just look at the treasure screen for empty spots. Again, no set order to get them, but some are behind obstacles, and you'll need to have certain forms to overcome those obstacles. Backtracking is inevitable anyway.
