Has anyone considered classes?
- Can build an exclusive sixth building in the mining tree which grants even more metal per turn.
- Armored units cost half the normal amount.
- Can build an exclusive sixth building in the oil tree which grants even more oil per turn.
- Travel costs 33% less oil for your units.
- Can build an exclusive sixth building in the conversion tree that lowers conversion rate by an additional 20%.
- Increases metal and oil gain rate (shown as a bonus. Almost what Oil Tycoon and Mining Tank gets, maybe 33% less for balancing purposes.)
You would pick your class by building the sixth building in the tree. You only get one choice, though. Instant build instead of waiting.
EDIT: Give it a 24 hour cooldown if changing classes to avoid too much donor spam. Possibly charge reds to change class.