There's one more (optional) form to get - if you check your map there will be an unvisited room in the mithalas cathedral. (You need nature form to open the blue/greenish large bulb plants to get there).
Also, have you been in the Abyss yet? That's where to go next. Down into the darkness...
If you haven't found the entrance yet, keep going down in open waters.
There's 3 things to do there, at this time: The whale (needs that form above), the sunken city (look out for a huge metal door, deep down), and the frozen veil (near the whale).
There's a scene after the sunken city that tells you where to go from there, but I recommend doing the other 2 places before.
