Can you add an option where the senator can choose to be a good emperor or dark emperor?

I'll consider that as a possibility for future updates.

Is it intended that the scry for capital location ritual can ONLY be used in level 2 libraries? I can't seem to use it on a level 3 library.

Nope! Thanks for calling that out, I'll get that adjusted soon so level 3 libraries are included.

So, there are 3 imperial cults. They are kind of barebones... The special princepes make it better, but even they are kind of low impact and situational. I propose a special ritual for each kind of priest, that each enchant a temple with a different effect. A temple can only be used by 1 cult! Rather than making them investments that commit the faction to a strategy, I think they ought to encourage strategic use by having very different effects.

Serpent Priest: Hatch Temple Serpent (75 G): Summons a temporary "Temple Hatchling," like a serpent but a bit stronger physically, and possessing Serpent Magic 1. When it levels up to 1, it becomes a "Temple Serpent," 4 tiles large, with Serpent Magic 2. Later on at level 2, it becomes an orm-like "Temple Tyrant" with Serpent Magic 3.

Heliodromus: Establish Temple Court (free): Promotes self to a Sun Judge. The sun judge has a +1 Gold and Trade bonus, with an improved version of the Heliodromus's "Enables recruitment" that gives improved access to Leos. Rarely, will freespawn a group of "Daywatch" who stupid around together from the temple. These are like Princepes Solaris, but stupid and with innate Awe.

Renatus: Open Underworld Gate(25 Gold): this enchantment makes a gateway in the temple, that leads to Hades. (Princepes Renatus are now hades immune by default) Freespawns a trickle of Princepes renatus.

So we have one ritual that protects the temple itself, one that offers wider area control and some modest economy, and one that enables hades conquest-making the dark emperor situation attainable in any map.

I definitely think expanding on the cults as a mechanic is a concept that could be worked with. I'll give that some thought.

This idea is meant to replace the "innate" archer mercs.

Auxiliaries: Three statlines under the name, (-1 morale) archers, and (-1 morale) crossbows, and a sort of “imperial arbalest.”
The arbalest would be slow, but have a strong attack.
Not sure if it should work exactly like the dwarven, or it should shoot little darts every round to feel distinct.

They would come in batches of 7, efficient but inconsistent to sell the theme.

I do have some new content already cooked up that lines up with this somewhat.

1. Archer mercs are out, more frequent "Auxilia Slingers" are in.

2. The "Raise Limitanei" ritual will now be available to the Senator right from the start, giving this commander one among a few new reasons to actually leave the starting citadel prior to gaining the capital.

3. The Hero will be gaining a new quest that can unlock a "forgotten imperial cult" of sorts that will enable merc offers for archer legionnaires.

Spotlight on the ritual "Claim Magic Spear and Shield." On some maps, this can be quite something to pull off.
I would recommend adding additional tile types it works on like, like the fortified player faction citadels, and things like the Academy of Higher Magic, Desert Palaces etc.

Yeah, that's a fair point, obtaining a "Forge" location in some ages can basically amount to finding the capital itself. It could probably be expanded to a more generous terrain group.

New ritual on the Renatus: "Oath Guard." Can only be cast once per Renatus. Can only be cast in Hades. Summons a group of undead legionnaires. Costs 75 Gold.

A cost-inefficient but clutch way to support a renatus to reach water and/or seize poorly guarded resources.

New Ritual for Serpent Priest: "Syncretic Mission." Meant as a small side-bennie for the serpent cult rather than a spotlight draw, this can only be cast on Serpent Tribe Villages. Cost 50 Gold and enchants the tile to freespawn troops. What troops though?

"Serpent Tribe Warriors" might actually fit ok into a standard Senator stack that uses one big javelin filled front line. If you were going to make new "Serpent Auxiliary" poison bows would be a draw.

Given that the serpent cult will have an increased presence beyond an occasional caster recruitment, I think a small minor interaction with Serpent Tribe Villages could be fun. Same can be said for High Priestess and Spider Tribe Villages.

The Renatus concept is decent. I wouldn't want them to get to a point where escaping Hades is a cakewalk and basically gives them pseudo-immortality, but the caveat of it being a "once per caster" ritual does cover that, so it's worth consideration.