To create? Or just one to download and play with some friends later? I think it's a bit late to be starting on a mini-game for H3. I'd say way till Reach to get started on a new idea. Actually, you could make a mini-game, but you would need to test, tweak, test, tweak, cont.
Look for popular games in everyday life. Whether it be some game in an arcade, or an actual physical game. From there, see if you can use Halo's mechanics to construct it in forge. From there, to make it memorable, add something in it that pushes the limits of what Bungie intended to see with Forge. That could be a switch, or using the map geometry for an alternative purpose. Make it something new, original, and fun.