In order to make r6 ing units and earlier game units in general more desirable, it'd be nice if we had a building (potentially +pop adjustment to accommodate) which would allow us to promote, or "elite," an r6 unit to a better version once the unit type has reached r6.
The promotion process would require time and resource as though training a unit, but, similar to infecting units, it would also require a unit, the main difference being the unit would need to be r6 and the promotion/eliting cost would include t3, possibly t4, and/or even new resources designed for this mechanic. It would also not be random. Other prerequisites such as a level requirement or specialized buildings (gun foundry, rocket factory, armor shop) could be used.
The promoted units would only be available through this method and they ideally should be level scaled similar to the TF2 units to keep them somewhat relevant.
Both units would have the same move pool to keep the feel and niche of the unit intact
This method could also be applied to Infected, going from proto-normal-advanced-archetype.
It could also be how players obtain the Brutal Raiders should Madrona wish to implement them as player-controlled units.
Why should this exist? To further reward players for ranking up units as well as adding more content into the game at most levels of play.
This would also allow players to have access to a wider variety of units and has the potential to breathe life into underused units.
Battle Nations has a scaling problem, and a suggestion such as this, if implemented and balanced well, could help with that by allowing more units to remain relevant at more levels