How would we be able to build our own champions if we cannot mod existing ones?
Do you mean champion paths?
You can mod existing paths, at least to an extent.
You can create a completely new path.
You just can't (currently) block a path, or restrict it to a specific faction/race.
And you also can't have an ability that is only available if you have a particular faction trait. For example, take the "Tinker" ability on the General tab. Let's say you want to make it so that the Tinker ability is only available to Altar... Or it's only available if you have faction trait X... that doesn't work as far as I can tell.