that sounds super fun! i would like to help!! I have been planning to some day in the future to do something like that but more of butlers and lolita maids.. but its all the same as long as we try to let our customers have fun. I have a degree in Baking & Patisierre and I do have cafe experiences via serving and great customer service (working in nordstrom's cafe).
anywhoo, why not do some japanese based food since it will be a "cosplay cafe" and maybe even sweets? maybe you can based food off some animes? for example Naruto's favorite ramen dish? or maybe a dessert buffet?
if you're thinking sweets..
mini fruit tarts
jumbo size individual creme puffs
or even cute plated desserts, possible with anime characters drawn with sauce or something?
*i made these too so yummiess*
the thing about having it in portland is the Location. Sometimes the most secluded place is the best. like Rimsky house is a dessert house only open at night and its popular. Its good to have a business plan made up first. usually about the finances and how much you would spends per year. usually new business dont last long if they dont succeed the very first year. and of course you need to know the design plan and the menus and everything you are serving and hmm. there is alot put into it. hopefully I can be of some help. I know I like to be a helping hand as well!