Field Type = Neutral
Complexity = Moderate
Format = Eh, whatever
No Restrictions on anything
Here at the magical type shop, Mr.Heefloot sells many strange and  mystical devices. Within these seemingly ordinary devices, a great power  is contained, so great that it is even capable of changing the types of  Pokemon! For a meager price of less than nothing, Mr.Heefloot is glad  to rent these items out to budding trainers for the duration of a battle  in his yard. He just loves seeing small creatures crushed by Shieldons!  His yard is rather regular, covered in grass and with a small pond  nearby, so there are no move restrictions.
The arena is nothing special, but Mr.Heefloot will gladly give one of  his precious seventeen items away to a Pokemon at the start of a battle,  adding an additional type to them! For example, if the Fairy Wings  are given to a Shieldon, then it would be counted for purposes of STAB,  type resistances, etc. as a Rock/Steel/Flying type! Just to clear some  stuff up, Mr.Heefloot says a Rock/Steel/Dark type is considered 8x weak  to Fighting attacks (for an ASB modifier of 3.0), and a Water/Ice/Fire  type is 8x resistant to Ice attacks (for an ASB modifier of .29). In  addition, a Pokemon may take an item of a type they already are. For  example Conkeldurr can go ahead and grab a Super Karate Black Belt  to gain double the STAB bonus on fighting-attacks, but also doubling  his weaknesses! Only one of each item can be attached per battle, and  they do not take up the regular hold item a Pokemon usually has. Here is  the list of items, have fun incorporating them into your flavor refs!
Exceptionally Ordinary Something-or-other: This something-or-other has a habit of somewhat normalizing  the Pokemon that somehow manages to wear or use it or something like  that. Nobody really knows, its just kind of really ordinary and regular  and stuff.
Super Karate Black Belt: This black belt is extremely empowering, and instantly gives any Pokemon that wears it the fighting prowess of Jet Lee, Chuck Norris, Yip-man, and Sawk combined! "Be wary of flying feet and fists," it says on the genuine label.
Fairy Wings: Though these wings may just look like they were  ripped off of some five-year old's Halloween costume, they are genuine  fairy wings. Easily super-glued on to even the most resistant of  Pokemon, it grants them the ability to soar around like the average flying-type.
Hive Mind: A small and insectoid brain in a jar, the trainer  applies this item by deftly lobotomizing their Pokemon and replacing  their frontal lobes with this. The Pokemon is then conquered by a  strange alien hive race, and begins acting far more buggy than usual.
Rotten Flower: With on whiff of this disgusting flower, any Pokemon becomes capable of spewing out toxic, poisonous goop like no other. It really is that disgusting. Trainers are suggested to wear gas masks around this item.
Prickly Rose: To apply the powers of this item, the trainer  simply slaps their Pokemon silly with it. Eventually, the thorny flower  will cause a puncture wound, and its grassy juices with enter the  Pokemon's bloodstream. Often, the Pokemon will have a habit of lying  around inactively in the sun shortly after being Roseified. Studies are  inconclusive on whether this is due to photosynthesis or blood loss.
Scuba Flippers: Just stick these finely-crafted flippers onto any  Pokemon's feet, and they'll be swiftly swimming through any drizzle!  Instantly gain access to water sources like never before! Note:  not proven to work on Pokemon without feet; Mr.Heefloot suggests jamming  the flippers on the Pokemon's head.
Torch: Light up your favorite pal, and voila! Instant fire-type!
Mad Scientist Goggles: Equip these stereotypical goggles to a Pokemon, and watch as their fur/hair/scales/shield-faces becomes instantly frazzled by electricity! Soon, they'll be conducting dangerous experiments with Frankensteins and Tesla coils! They grow up so fast...
Refrigerator: Often used in haikus, fridges apparently have  another use! Stick this smaller version on top of a Pokemon's head, and  they'll be calling Blizzards down upon the battle. It's a wonder what a  little bit of chilled ice can do for someone!
Bastiodon Skeleton: Unfortunately, not all Bastiodon fossils were  in a good enough condition to resurrect. So, Mr.Heefloot hoarded them,  and will gladly preform a rough full skeleton transplant with any brave  Pokemon. The sturdy bones of the tough Bastiodon imbue the the patient  with rock-hard skin.
Model Train (1:2 scale): When a Pokemon conducts this half-size  train, they barely even need their own powers anymore! Honestly, do they  really need to use wimpy Pokemon moves when they can just crush their  opponent beneath a steel train?
Shovel: As Diglett and Dugtrio show, all you need to be a ground-type is live halfway underground! The shovel makes this unusually tough change much easier.
Gypsy Tent: A Pokemon walks into this gypsy tent a simple Pokemon, and out comes a mystical psychic mind-reader, fully equipped with spoons, crystal globes, and tarot reading skills.
Rusty Ghostbuster: Even Mr.Heefloot doesn't know he managed to  acquire this antique, but he does know how it works. Just give it a  little kick, and out comes a wicked ghost, eager to posses the nearest Pokemon!
Ominous Hood: When hiding behind this dark, shrouded hood,  a Pokemon seems to become evil and wicked, no matter how nice it  usually is. It just seems to bring out the worst in everyone. Extended  exposure is not suggested.
Master Ball: Everybody knows that legendary dragons belong  in Master Balls! So, Mr.Heefloot will gladly allow your Pokemon to sit  in this legendary pokeball for a couple minutes, giving them the power  of the dragons that used to call it home! Note: Mr.Heefloot does not  guarantee the release of your Pokemon, especially if they are a  well-trained strongmon.