Rotom's Basic stage - Would act as a light bulb so it would be able to light caves and dark places when it is thrown out.
Rotom Frost - Would act as if you had the frost walker enchantment. It would freeze the water for a short time when trying to cross it.
Rotom Heat - Cooks apricorns in your inventory. (Would need some balancing but something like 1 apricorn every 2 minutes would not be bad.)
Rotom Mow - Would cut grass in the area it moves, If player own'd and thrown out of the Pokeball.
Rotom Wash - Could use once every 20 minutes to clean a fossil if it is throw out and right clicked with the fossil in your hand.
Rotom Fan - Could not come up with something clever for this but I am sure you guys can figure out something for this one.
Thank you for taking the time to read this and I hope this spark some more great and wonderful ideas.