Now I know what literally everyone is going to say, "it'll be pointless as it's only cosmetic and it will just slow development times" and I guess you'd be right but keep in mind this is a suggestion not a "please add this right now".
Except more customization like this is good because it gives players something to do and something to enjoy collecting, plus with more items to customize ourselves with can make players more unique so here:
The particle effects I think we should have as equip-able are already in the game,
I'm going to format this list like this
Particle Effect we should have -
what already has it
Confetti Particle Effect - Confetti Gun, Party Crab, Celebrations Shop Keeper
Spooky Particle Effect - Spooky Gun
Bubbles Particle Effect - Bubble Gun
Money Rain Particle Effect - Unit Bills item (maybe not this one as it's a backer item)
Party Lights Effect - Party Crab, Celebrations Shop Keeper, Bowling alley
These are the only ones I can name off the top of my head but I also think the weather effects like snow and stuff could be very nice particle effects.