Are you proposing that each mentat should have one specialty but it affects all cities, and each civ should only be allowed to have one mentat?
Close. Two effects, that may or may not affect each city, based on one unit.
But now that you mention it, it might be a better idea to have the promotions be like this:
One part of the promotion is "hands on" for say, combat/spying/diplomacy, and the other is civ wide.
Example:
You decide that the Mentat is going to be your spy master. The unit individually gains a large bonus to all spying it does, but it also increases spy defenses by 5% as long as the unit lives.
Different upgrades could be:
Weaponmaster: +10xp for Mentat combat upgrades, +1xp all units produced everywhere.
Philosopher: +city happiness, +production speed of all missionaries/temples
That way you have to balance protecting the unit with using its special/in hotspots.
These seem very similar -- actually identical in many cases -- to the existing Mentat abilities. Did I misunderstand something?
Yes similar ones exist already.
I was using them as basis for examples as civ wide effects instead of per city.
I was thinking that if you affect everything, you have to be more careful with effects.