How about keeping the local population spells at the hero and replacing the global ones by a national project (which costs less production as you gain the later techs)? National project means repeatable but only 1 per player at a time.
does this need python work, because i can't make a ritual that is repeatable but can only be build in one city at a time?
I will go for the Ritual route mainly because there can be more players playing as the Aristrakh without problems and the ai shoud be much better handling the rituals then the spells, meaning it should not be that much of a problem for the ai to play as the aristrakh, so could even make the ritual death's embrace (the one that changes the calabim civilization into the new one) available to the ai.
Edit: I am theory-crafting at the moment.
There is a problem with the local version (Death Pact) at the moment:
-In the late game (or to be more precise in cities with a good production) it needs to be expensive but adds several citizens, this is because you don't want to micromanage your cities and assigning the same project every turn is boring
-In the beginning however it needs to be cheap, since you will need this as a start off and get your game running.
My solution was that higher tiers of the spell add more population but it would also cost more. The ratio cost/population-gain would decrease so it would be worthwhile researching.
This brings me to another problem however: let's assume the player still founds new cities in the late game then he would have a problem since the local version would take far too long to complete in a small city, he has to use the global version to kick it off.
I could circumvent this by not letting the higher tier rituals obsolete the lower tier ones, this would clutter the screen however.
The best solution would be if the ritual would be not only dependent on the technology, but also on the city size. If this could be implemented you will always have the best option available and the screen would always show two rituals available at maximum: the global and the local one.