online. lots of levels,weps,spells, and things were it actually requires skill. but also things thta need teamwork,effort,time. also graphics make a game. and one of the biggest things (especially for online. NO LAG
make sure its challenging, or people get bored to quickly. make sure its not ridiculously hard (unless its the unfair platformer) and MAKE SURE IT HAS GOOD GRAPHICS. and people hate lag. oh, and make more than one game mode. like a story mode and, hmmm...
oh yeah, if its a shooting game it can have a practice range. like, stand somewhere safe and blow everything up.