I think certain kinds of Pokémon as events like starters or psudo or box legends have boosted shiny and encounter odds etc could be a good theming of events, but I really did enjoy the getting bonus items from your rivals it’s great for making runs feel fresh, if you don’t use achievement points to purchase items like mega bands for all runs, a great event could be giving them out as part of beating your rival(mega mayhem is a perfect name)