Rotom, the Ghost/Electric-type introduced in Generation IV, is a very strange but interesting Pokémon. Able to "possess" various electrical appliances around your home, Rotom can actually change its typing based on which item it is inhabiting.
For example, when possessing a washing machine, Rotom takes the form of it and becomes Water/Electric-type. Since its introduction, Rotom hasn't been given any new forms to use in battle, but it has been seen possessing some other devices. In Generation VII, Rotom possesses the player's Pokédex, and in Generation VIII, it seemingly inhabits a bicycle. Here are some other devices that Rotom could inhabit if the developers decided to add new forms.
10 WiFi Router
The first item that Rotom could theoretically inhabit is a WiFi router. How this would affect an internet connection is unknown, but it would hopefully boost it and make it more stable. No one likes a spotty internet connection!
In this form, it would make sense for Rotom to be Psychic/Electric-type. The WiFi signals being produced from the router would be like Rotom's brainwaves. How interesting!
9 Soldering Iron
The most obvious choice for a Steel-type form for the ghostly Rotom is a soldering iron. Soldering is a process of using a melted metal to adjoin two pieces of hardware, other metal, or various things together.
In this form, Rotom could make use of some interesting Steel-type moves. Something like Iron Defense or Metal Claw would have interesting appearances if it were a ghostly soldering iron performing them. Melting metal to use for an attack is terrifying to think about.
8 Arcade Game
The glory days of arcades were filled with intense gamers who would try to fight one another with various martial arts-based games like Mortal Kombat and Street Fighter. These arcade games went on to become home console games and still hold a special place in the hearts of many.
If Rotom were to possess one of these arcade game cabinets, it would make sense as a Fighting/Electric-type. It would be hilarious to see the little pixel fighters throwing their 8-bit punches at one another.
7 Blender
The Pokémon Company seems to love the idea that food-based Pokémon are Fairy-type. Just look at prominent examples like Alcremie, Aromatisse, and Slurpuff. If it's made out of dessert, it's apparently a fairy.
This could be used for a Fairy/Electric-type form for Rotom. Just let it possess a blender! A pretty pink smoothie inside that blender would make obvious sense as a Fairy-type. How perfect would the move Aromatic Mist be when used by a Fairy blender?
6 Wires
While all of Rotom's forms still retain the Electric-typing, none of them are pure Electric-type. This could be done in a very obvious and simple way: let Rotom possess a bundle of copper wire. These wires are used in almost all electronics, and they carry the currents from point to point.
This form could also be Steel-type, given the copper-based nature of these wires. However, the Ultra Beast Xurkitree is basically a bundle of wires, and it's pure Electric-type. The same could be perfect for Rotom.
5 Jackhammer
There aren't a ton of devices or appliances that are electrically-powered and also made of rocks, but a fun choice for a Rock/Electric-type form for Rotom would be a jackhammer. These large, loud rock-breaking tools are mainly used by construction workers, seemingly to drive anyone living nearby mad from the noise.
A Rotom-possessed jackhammer would be an intimidating foe. Making use of moves like Rock Wrecker and Head Smash would be just perfect for a Rotom in this form.
4 Bug Zapper
One of the most interesting type combinations that Rotom could take on would be the currently-rare Bug/Electric-type, which is only used at the moment by the Joltik and Grubbin lines.
Rotom as a Bug/Electric-type might work perfectly as a bug zapper. Sure, these devices are meant to kill bugs, but it would still be fitting. When turning into this form, Rotom could take the move Bug Buzz, which is comically fitting.
3 Keyboard Piano
It's hard to choose which device would be the best for the currently-uncommon Normal/Electric-typing as a Rotom form, since there are so many options that would fit. However, a fitting and fun one would be a keyboard piano. Alternatively, another instrument would make a good choice as well, like an electric guitar.
Normal is often associated with sound and music since there is no Sound-type. Chatot and Exploud, among others, are sound-based Pokémon that are Normal-type. The same could hold true for Rotom as well.
2 Vacuum Cleaner
There aren't many ways in which a dragon could be associated with a household appliance, but a vacuum cleaner might work, just for fun. The air suction and physics-based nature of vacuums are the same types of phenomenon sometimes associated with Dragon-type Pokémon.
Alternatively, a Rotom possessing a vacuum cleaner could retain base-form Rotom's Ghost/Electric-typing, since the vacuum of space would be somewhat tied to the darkness of a poltergeist and ghosts. Either way, it would be a fun appliance for Rotom to take over.
1 Coffee Maker
There's nothing inherently evil about coffee, but a coffee maker inhabited by Rotom could adopt the Dark typing. This could be matched to its design if the Rotom were shown being hyperactive and evil, using its scalding coffee as a weapon.
Coffee beans are dark in color, and many coffee makers are black or dark-colored. This could be used by an artistically-inclined Pokémon designer perfectly to make an evil-looking coffee maker, similar to the evil-looking lawnmower that Mow Rotom is. They made a chandelier look evil, after all.