It has been mentioned before, but…
It would be nice to have a ‘Challenge Yourself’ button in the ‘Deployment screen’.
After you deploy a fleet this button would remove the enemy fleet on the right of the screen, place your fleet there and allow you to deploy a second fleet to fight your first one.
This would make it simpler to test your challenges before posting them online, pit your ship-designs against each other, and give you some challenge functionality even when you are not online.
The same can already be done by challenging yourself in the current challenge system, but that requires several more steps/screens to go through than pressing one button. It also requires you to be online.