There are so many, MANY, great effects in TF2. A lot of effects in game can be combined with the Plumbers Cap hat (cosmetic) to create references with Mario games. Over time, Super Mario, (instead of becoming one of the most well known characters in gaming and a mascot for Nintendo) had many opportunitys and video games based on this iconic character. Super Mario, instead of being a character in a platformer (being one of the most innovated games at the time), had changed and been into different genres of games, such as: racing (Mario Kart series), sports (Olympics, Superstars, Mario Golf, Tennis series) 3D platformer (Mario Galaxy, Odyssey, 3D World series).

In short, Mario is one of the most influential video game characters to date and has been in many great games. With the combination of TF2 Unusuals and the Plumbers Cap hat (cosmetic), we can create many great combos and themes that references Mario and his video games, hence why the Plumber Cap hat (cosmetic) is the most themed hat (when it comes to Unusuals).

Here are the list of effects that can be themed with this hat.

Confetti = Mario Party
Nuts and Bolts = Metal Mario
Orbiting Fire = Fire Power Mario
Orbiting Planets = Mario Galaxy (Super Mario Galaxy)
Haunted Ghosts = Luigi's Mansion
Burning Flames = Mario Fire Power
Orbiting Eri Flame = Dark Mario (Evil Mario)
Sunbeams = Sunshine Mario (Super Mario Sunshine)

Thats about it, The list will be updated when I find more Unusuals that can be combined with this hat to create more unique themes!