Just to throw out a non-shapeshifting example, how about a Yogi or similar meditation-based "psychic"? The equivalent of a Shadowrun Physical Adept. He has a certain amount of... "Focus", or "Mental Strength", which he can focus each day by meditating with the sunrise. But depending on exactly how he spends his time meditating and what he focuses on, it manifests different. Today, he's super-strong, tomorrow he's super-fast...
Or an "elemental self-infuser". He can hold only so much raw elemental power, but he can use that power to infuse different aspects of his physical abilities. Wind = Speed, Earth = Toughness, Fire = Strength, Water = Recovery. Water could also be Aquatic, Fire could also be Breath Weapon, Air could also be "wings" (in a non-literal sense). Maybe throw in the catch that "Marked by Power" is required whenever you have any power infused, because, well, you're friggin' filling up your soul with raw elemental might. Oh, and the other catch would be you're vulnerable to opposed elements.