Not really too planned in advance... I think of a playmode or level design and then experiment. Sometimes it comes out nice and sometimes it sort of has a flaw or doesn't work in some way. Usually I try to fix stuff like that and find what can make it fun, but sometimes it's a lost cause. In the case of Killing Spree mode it took me years to put that together in my brain as something to try and it turned out quite fun and it gets played quite a bit too.