I like the current Health system.
A size 10 city feels like a big one, it is a fun change.
It changes from other games were you weren't satisfied until all cities were at least size 17. (20-25 for the top 4).
The only issue is that there are not enough
bonus late game (like in buildings) to help you have big cities.
I'll really like a system using the cathedral mechanism of vanilla civ :
like "Hospital" :
needs 4 cities with apothecary (or infirmary) (sorry for the spelling) :
+2
for life mana ; +2
for reagants, +2
for nature mana; +2
for incense.
Thus
for 1 city out of 4, you'll be able to get +8 health. (4 more pop)
Maybe kuriotates could get a special UB for Hospital, needing "only" infirmary + apothecary + library in city, but not needing to have mutliple cities requirement (as they have few cities). The UB could also have a +2
for ... fruit of Yggdrasil and creation mana (so they can get more pop in their sprawling cities)
Add another building "dietetics" :
buildable by Great Healer : +1
for fish/cow/corn/wine/banana/mushroom : potentialy +6
(+3pop) (one per "improvement-type")
those are random ideas, feel free to change name/specifics..etc but they would help you to :
-get a real use of great healers,
-still be able to get some "few" cities bigger than others, but not ALL your cities cold become really big.
-keep the early game as it currently is but allow for some late game big cities.
PS: sorry Opera, I didn't meant to annoy you
. Maybe you already thought of some fixes