With D274, Promotion can be made to add modifiers every time a unit gains a "level", instead of promoting to a different entity.
This is great for RPG or tower-defense mods. This allows creating entities that can gain a small amount of HP every time they gain a level, for example.
The system is designed with some flexibility, one can designate "ranges" where modifiers can differ (see tests).
With a bit more work, and a few more details, this could replace the existing Promotion->Entity system entirely, and would allow removing the Advanced and Elite technology hacks.