Useful abilities we've seen from Marco that he didn't use during Marineford:
- Healing others/covering a large area with his flames
- Using his flames offensively (shockwaves/blasts againsts Big Mom, King, and Queen)
I don't think Marco has a hybrid form other than transforming his arms and...
Marco fought admirals.
Marco fought Blackbeard at a disadvantage.
Marco fought fucking Big Mom.
Oda literally called Marco "incredibly strong" in an SBS.
Marco is currently making two calamities bleed at the same time.
"HaHA but ApOo alSo did that."
Marco's ability is really, really busted.
I'd even bet that Marco once tried to fight Whitebeard and, even though he was getting beat the fuck down, Whitebeard was probably like "Jesus I can't even knock this dude out wtf! HEY wanna join my crew?"