I've got some ideas (again):
Tahu: Lavasurfing IS better. I picture Tahu standing near a stone edge next to a lava pool. You press the special ability button, and he cuts off a part of the edge, which falls into the lava, Tahu jumps onto it, and it's time to surf.
Lewa: If the "second jump" in the double-jump is some sort of a power jump, as pictured on some of the cards mentioned elsewhere, it would be awsome!
About Gali: Yeah, they should all be able to swim ('xcept Pohatu), only they should be able to swim without Kaukau. Diving, on the other hand. Pohatu should be able to jump into water and walk on the bottom when wearing a Kaukau, or to get across water, he just power-runs over. When it comes to what Gali should have as her ability: I just remembered that in an early BIONICLE comic book, she said she could create rivers (or maybe creeks or something) anywhere, maybe this could be an option? Or maybe she should be able to make it rain? EDIT: She said, Quote: "...where there is moisture, I can make a flood!" so, something like that.
I hope this will (also) be of help. - dann494