So our 2 person team, my son and I started a day later on this and have just finished our second day.
First day we came up with 3 ideas - which when put to the focus group (mum and siblings) surprisingly all scored near similar scores. We discounted one idea as it would take too much development for the game AI. So it was really a toss up between the other two ideas; we went with "Zombie Bash" and the other game idea "Shatter" is up here on this forum in case anyone is stuck for ideas.
We're both learning Unity and having never used it before it's taking a while to get up to speed for us both. Seems like Unity really wants to work in "its own way" and trying to be more flexible with it isn't easy for beginners, so we've had to rework a lot of early code to be "Unity Friendly"; there's probably ways to get to work the way we wanted but as we're short of time we don't have the luxury to experiment. All that said we have finally got our zombies moving in the game - cue cries of "It's Alive! It's Alive!".
I have also shelved my idea of writing the main game code in C++ as the interfacing to C# for Unity would just slow us down too much when debugging. So it's now all in C#. I'm happy with that as just learning Unity will be a win for me.
We have recruited a couple of artists to draw us some backdrops for the game as it'll need many of them, it helps to have younger brothers. They have agreed to be paid in Overwatch Loot boxes :)
Here's the first draft of the backdrops for the first 2 levels:
Tomorrow we're working on player movement and animation and collision detection.
Having fun and learning lots so far...