I can suggest doing a MYO event other species do whenever they first open!
Offer a common slot if they share a bulletin and tag friends, and that helps with promo.
As for other type of events, do consider offering a gachamal currency for any completed prompts! If you are planning on opening a discord, consider implementing a #work channel. Those help with engagement on discord!
As for monthly prompt ideas, any that help users develop their own gachamal like “show your gachamal doing their hobby” or holiday/seasonal prompts are fun too :3
If you don't have plans for species currency (they can be tiring to keep track off if you are not used to it), consider raffling trait-upgrade items or even one myo slot for all of the prompt participants ^^
- For other type of events: draw-to-adopt, draw-the-mascot to enter a MYO raffle, design a new trait or pet to get prizes, gacha-based adopts, draw-the-character-above (easier to do on discord)