Making my game:
I originally wanted to make an RPG similar to Chrono Trigger (In the battle system), but found that no one on the "team" (A few friends), myself included liked that type of system. The ATB system just isn't that great, and I was the only one that wanted to do a turn-based RPG.
The story revolved around one person who was sent to "Earth" from "Heaven" to "Help seal away a Demon", who met up with another fighter from "Heaven" who was sent to stop him and his boss, and is aided by another person who may or may not have been his love interest. (There's a LOOOOOOOOOOOOOOOOOONG Back story for everything in quotes)
The whole draw was going to be, you're in control of all 3 people at the same time, but my team wanted to turn it into a real time RPG (a la Star Ocean: Last Hope), which ended up turning into a God of War like Hack/Slash with RPG elements.
It didn't help that we tried to split up the level design between us (We'd all make different set pieces). I made the tutorial area with optional paths, and a few secrets, that still guided the player towards the exit.
My friend made a box with a staircase in the corner, and when I asked him about the rest of the level he got pissed and said that his level was better than mine.
After that I played Dark Souls, and wanted to make a game similar. Not difficulty wise, but style-wise(Think of the "slow" combat and the focus on dodging/blocking as opposed to simply hitting for high numbers). My team agreed to this. So I took my level layout and carefully placed enemies. While My friend took his and basically turned it into a hallway with rooms on each side. He basically made the layout to a hotel and called it a day.
We also made boss designs (mechanics at least): I came up with a rough sketch, a move list and explanation of each move and how the player is expected to counter it(If you're curious I'll post it some other time, since this is already running long... Rants, eh?). He came up with "He throws enemies at you", "Attack him until he's at 50% HP", "He throws more enemies at you" (Kind of like The Joker in Arkham Asylum)
It felt like we were working on two different games, but with the same story, setting, and characters
EDIT: We have other projects too. For example, this horror game we're working on. He wants my help to design it, but when we have time he decides, "You can't help design it, you're going to test it. If you help make it, it won't be scary for you."