Why not just add 1 more promotion layer? on top of rank 3? So, champs will be offcially rank 4 unit. This can be done as following.
Boost the stats of the units same as champ, when reach rank 4 (No need to change their unit type as champ (Citizen to champ), just give them costume of champs)
or
Convert them to champions (Which is not suppoted I guess according to @borg-, but solution can be looked into by dev team)
I think first one should be widely acceptable. Since, these units will earn the position of the champ and can't be converetd to champs by just spending some resources. I personally don't link the idea of converting normal units to champs just by spending res becasue of 2 reasons.
It promotes passive gameplays, where player will just gather res untill he can upgrade his units
Not making existing champs at all. Why spend resouce in making champs (that doesn't do eco), when you can train eco doing un@#$% which can be made champs.
There are other reasons but I don't want to create a long list but you get the idea.
Experiece based promotion should be supported. This will making gameplay intereting in many ways and its a fair gameplay.
People will focus on promoting their units, so they will take fair fight rather then go and suicide.
Units fighting for so long with so much exp won't be slaughtered by few champs.
People will use healing better, since you don't want 1 Hp champ.
Its just better gameplay, if you are microing your units till rank 4, you should be rewarded.
Thoughts @borg-, @andy5995, @Grapjas, @Boudica, @ValihrAnt, ?