No, there is no Sandbox. 4X RTS games typically don't have this because of how useless it is.

There would be nothing to do, no units would be required if there's nothing to kill and consequently no buildings would be needed.
There's no practice to be had as you are up against nothing to practice with.

If you are desperate enough for this, it is possible to make a map and have a blank script made which will remove all the rules for the game and allow you to do whatever you want.. with exception for any combat. *shrugs*

Not sure exactly what you mean about creating own units, you can train units with any of the weapons you've researched. If you are talking about creating models of units, that is impossible.
It is however possible to mod units, such as their training (cost/time) plus the units properties (Speed/hp etc), using the Scripting tool found in the Map Editor, this would only apply for the specific map it's made for. Currently you can't mod weapons but that's a future update.

Research consists of 7 new weapons - useable by most units (not infantry, Vanguards or Bombers) which allow for over a hundred combinations.
And then another 6 (I think) Research to upgrade units and commander abilities.

The only way to win - except for scripted maps - is for the 4th X.
Exterminate.


Just to point something out, you posted this under Workshop Discussion as opposed to the main forum for the game, slightly out the way.