So, if your key cards are all expensive, you likely need to include some cheaper cards that help build to those other cards and vice versa
Make people play at your speed
Ive been consistently surprised by Stonecoil Serpent in the Arena Cubes, and its a card that falls within the realm of playable even in Vintage Cube, though merely being an artifact is doing a lot of the heavy lifting there
It could be that Universes Beyond-related star power is lifting the value - Black Flag is one of the best-liked AC games, after all
Especially while Invasion block is around, you might as well go crazy with the number of colors you use
Flamewar, Brash Veteran Flamewar is an interesting commander because when I saw him, the first thing I thought about was Prosper, Tome-Bound