Sounds like the kind of thing that will kill a project if left unchecked. He very likely doesn't realize that he's coming off as negative and controlling. You should sit down with him and talk it out -- ask him to be less rigid about his vision, and ask about the kind of game he's looking for. After all, you're a co-designer, not a beta tester for his game.
On topic: Let me spitball a few ideas, since the topic seems to be getting bogged down.
We've agreed that the game should use upgradable dice, and we're also interested in randomized events and leveling character classes. It makes sense for the die advancement to be rolled into the character advancement. Mown's also raised the idea of drafting dice and using them as a resource, which would give us a turn-to-turn resource pool that grows over time. TPmanW's idea of a combo system could be used as well.
What kind of game works well with an incremental resource pool, dice-based resolution, and combination moves? This might be low-hanging fruit, but I think our mechanics are a good fit for the theme of
combat. I like the idea of a multiplayer turn-based combat game. It has a lot of potential depth, as we've seen in the many varied mechanics of dungeon crawlers and MOBAs. It's also flexible enough to be either cooperative or competitive depending on where we choose to place the conflict.
What, then, would our character dice translate to? What resource would fuel your various moves? How do we mash together all these disparate ideas? We have different kinds of dice and a way to upgrade them over the course of the game. We want to draft dice. We want to be able to use the dice in combinations. And this is just me, but I'd like the game to use standard dice, just as a concession to convenience. So let me propose this:
Dice in our game are six-sided dice that represent
mana. There are multiple colors of dice, each with a different special quality, and you can also make specific combinations to perform extra powerful moves.
Our character classes are different types of
magically empowered fighters. They each have an
internal reservoir of dice, usually skewed towards a few colors, and which can be expanded over the course of the game. Meanwhile, the
winds of magic blow randomly at the start of each combat round, and can contain any combination of colors of dice. Combat revolves around drafting the right dice from the winds of magic to fuel your moves.
What do you all think of this?