I know it's still a very early build, but here's my feedback so far.
-So, I thought this game could be the spiritual successor of Stuntman and its arena editor mode, and now that I've played it I was a bit disappointed to see the lack of placeable cars/trucks/stuff like that in the editor for some crash testing.
In relation to this, add a Crash Testing gamemode. More damage = more points
-The environment is also dull and white, so I hope there will be arenas, stadiums, ovals and maybe Beamng-style maps for more visual goodness and more gameplay variety (proof: https://www.youtube.com/watch?v=J2ddFxm2ags
In addition, the ability to add different terrains in the in-game editor, where you want.
-Different drivers are a welcome addition, but I prefer to choose cars and drivers individually and not limit one car to one driver.
-The awesome rolls from early builds of Wreckfest are still missing in this game too, so both Stuntfest and actual Wreckfest version need physics improvements
(especially Wreckfest, since it shouldn't have any sort of assist that tries to save your car from flipping over).
-Races would be a good addition in Stuntfest too. Arcadey/chaotic races through different gamemodes, and the ability to create AI paths in the editor. This game totally needs that!
-Last but not least, will there be mod support for placeable props (including destructible vehicles) and maps?
EDIT: even more suggestions
-Slow motion feature (possibly for track testing, free roam etc.)
-More props from the Sneak Peek 2.0, and more props from the Flatout games (soccer, rings of fire, etc.)
-Ability to look around using the mouse, both in driving mode and ragdoll mode.
-Some sort of multiplayer mode would do wonders.
-In the editor, the ability to change animation speed and start time for animated props (so that not all animated props will be sync-ed to each other).
-Ability to change the size of props.
-"Vehicle damage" setting should also affect car deformation, mechanical damage and tires, possibly make them separates settings.
-Ability to place custom text in the map.
This game has potential, but do consider these suggestions.