+ Poison Heal + Knock Off
It is an alternative to garchomp who has longetivity and is able to be "immune to status" and is able to pivot around with utility such as being able to defog in front of heatran, and raikou which is pretty big as raikou pretty much makes deffoging a pain in the ass. This also may allow it to run a bulky dd set which can take advantage of its immunity to status and being able to threaten bulky teams.
Knock OFF allows you to provide support to your team with other moves and allows you to make sure you are able to force progress against the opponent
+ Slack Off + 15 Defense
hmmm... a bulky SD breaker... interesting... idk how i feel about this but sounds like a good mon against physical offense such as weavile, dragonite, and sd garchomp and its able to use those pokemon as setup sweeper...
+ Grassy Surge
252 SpA Rotom-Mow Leaf Storm vs. 252 HP / 252+ SpD Landorus-Therian in Grassy Terrain: 198-234 (51.8 - 61.2%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 SpA Rotom-Mow Leaf Storm vs. 0 HP / 4 SpD Garchomp in Grassy Terrain: 271-319 (75.9 - 89.3%) -- guaranteed 2HKO after Stealth Rock, Leftovers recovery, and Grassy Terrain recovery
252 SpA Rotom-Mow Leaf Storm vs. 0 HP / 0 SpD Zeraora in Grassy Terrain: 286-337 (90.2 - 106.3%) -- 37.5% chance to OHKO
This shows how powerful rotom-c is and its unblockable volt switches pretty much as nothing wants to switch into the powerful leaf storms in terrain. This also gives it a niche as a partner who can pivot in hawlucha for an end game sweep. It sounds like a mon that can fit on offensive teams such as volt turn, bulkier teams, and HO potentially. It can use NP, pivot, and scarf are potential sets that it can possibly use
+ Ground type (Replaces Rock)
steel/ground is still a bit awkward and i feel like it wouldn't do much especially since heat crash already hit steels hard enough and stone edge was able to nuke pokemon such as dragonite, golisopod, rotom-w. IDK... dunno much about it... gives it better defensive typing but i feel like offensive this feels worse
+ Trick + 20 SpATK
more sp. attack allows it to be a much better nuke... and can allow it to function on electric terrain teams actually nuking pokemon harder and making sure that the grounds are crippled due to trick such as lando-t, garchomp, swampert, and mudsdale