I have a game design that basically works. The technical issues are mostly sorted out, I just need to get a few more implemented before I can seriously play test it.
But fun? It's hard to know if it's actually going to be any fun to play. With a 4-player party-style game, it's seems like it might be hard to hit that fine line where everyone is close and competing, where everything feels exciting and tense, as opposed to tedious and boring. And despite envisioning my game as fun, it might just be boring to play.
Some of that comes down to tweaking it. Tweaking the speeds, difficulties, etc, will make a difference. (If it's too easy to shoot people from behind, then it will be nearly impossible to hold a lead for very long, which could ruin it and make it no fun. If it's too hard to kill the guy in front, it will be tedious the other way. If the barriers are too hard to dodge, then players won't have any attention left to fighting each other, which will be boring. If they are too easy, then they will be pointless, and it will just be a shoot-em-up)
So I'm really looking forward to getting a bit more done, and then conscripting some friends to come over and spend an hour or two playing it. I might need to have multiple builds ready that night, with slight variations in the timings and speeds and difficulties, to see which works the best.
The Theory of Fun and books about game design
I got a book out of my local library called Theory of Fun for Game Design which is (or is supposed to be) all about this concept. I was a bit disappointed. I thought it would give some good principals for coming up with a good game design. But it really didn't. It was mostly fluffy stuff about how the author thinks about games from a theoretical point of view, but didn't really have anything practical. We have another book in our library called The Art of Game Design which I read once years ago (it was checked out when I recently went looking for it), and is a MUCH better resource about this topic. I don't really remember much about it, but I remember being really inspired to try to make something fun after reading it.