[henshinzero]

Key items from previous:
W/Double:
- Gaia Memories & Gadgets

OOO:
- Medals & Drinking Cans

Fourze:
- Switches & Food

Wizard:
- Rings & Plastic Models

Gaim:
- Locks

How about...

1) Dinosaur rider series
- Style of Hulk & Tarzan
- Key Item: Disks

2) Constellation racer rider series
- Style of Saint Seiya, Speed Racer & Hot Wheels Acceleracers
- Key Item: Wheels

3) Dark Monster hunter rider series
- Style of Castlevania, Devil May Cry & Darkstalkers
- Key Item: Keys (Ryukendo style)

4) Ninja rider series
- Style of Ninja Gaiden & Rurouni Kenshin
- Key Item: Origami

5) Martial arts rider series
- Style of Mortal Kombat, Street Fighter, Tekken & Dragonball Z
- Key Item: Crystal Orbs