Wouldn't it be neat if while making a custom game there was also an option for either creating a new unit or customizing an old one?
This would allow a lot more creativity in games, for instance you could create units such as barricades, which would have extremely high defense, and be unmovable, or extremely offensive units with almost unlimited ranges, the choices are infinite.
You could click a box to whether the unit would be buyable or not. If not, then the only way to get the unit would be for the creator of the scenario to have put them in there him/herself, and would not drain the owner's income. If they are made purchaseable, then they would also have an option to set the price.
I don't know if this is even possible to create, but i wouldn't think that it would be very difficult to program. Obviously, the system could be abused when a scenario is made to give one side practically invincible units, but that can already happen already, and people just don't play that scenario. This would give the creators of scenarios much more freedom to create what they see as ideal.