Trigger: Any pagan lands (narrative event starting rebellion) after ruler converts to christianity (or other "stable" religion). Pagan religion must be majority.
Event:
Title: Pagan Reaction!
Text: [pagan religion name] in your country grew restless since you decided to became one of ["normal" religion name]. They feel that new ways you introduced are highly offensive to their customs. It's not yet rebellion but theres seems no way to avoid it!
Choice A: Deus vult! [or another corresponding text for other religions)
Big revolt risk to all pagan provinces mentioned in event (Slavs and Romuva won't revolt together for example). All pagan characters despise ruler (-50 to relations). +50 to piety, +50 to prestige. Non- pagan characters get big + to relations.
Choice B (ruler can't be zealous): Screw ["normal" religion name] I'm [pagan religion]!
Ruler converts to mentioned religion, gets + to relations with pagan characters and malus with non-pagan. All "normal" religion provinces get big revolt risk. -50 to piety, -50 to prestige. Pagan religion gains religious authority, "normal" religion loses it.
Choice C (cynical ruler): I can be [pagan religion] I don't really care.
Ruler converts to mentioned religion, gets + to relations with pagan characters and malus with non-pagan (smaller than above). All "normal" religion provinces get big revolt risk (smaller than above). -100 to piety. Both pagan and "normal" religion lose small amount of religious authority. Ruler gets "the pragmatist" nickname.
Choice D (zealous ruler): In [Allahs, Gods, etc.] name! Those heathens shall pay with their blood!
+100 to piety, +100 to prestige. Big bonus to relations with non-pagans, huge malus to relations with all pagans (-200), every pagan province revolts- if there was "normal" religion ruler, he's overthrown. "Normal" religion gets + to religious authority, pagan religion loses it. Ruler gets "defender of faith" nickname.
Choice E (just+zealous ruler): They're Gods childrens as we are! They only wandered from path. Stop persecutions!
+50 to piety, +50 to prestige. 50% chance of getting "humble" trait. Small malus to relations with pagans, moderate to relations with zealous (but not just) non-pagan rulers. Small revolt risk to all pagan provinces. Ruler gets "the tolerant" nickname.
Choice F (cynical + just): Religion is only a tool to persecute people. It shall not stand! Persecutors shall be persecuted now!
[most inventive of my ideas

]
Every temple holding is converted to city (25%) or castle (75%). Every bishop is banished to papacy (or its counterpart). -200 to piety, +200 to prestige. +100 to relations with cynical characters, -200 to relations with zealous characters, -50 to all others characters. 50% chance for revolt in every holding formerly with temple (new revolt system allows non landed characters to lead these revolts. If they win, they become bishops and gain whole province). Every mentioned religion loses a bit authority. Ruler gets "the atheist" nickname.
And "bonus event". It probably doesn't have even small chance to make it, but it'd be amazing if it would

:
Trigger: One of Popes vassals (no other character). Decision.
Event title: The very old gods. [narrative]
Description: Shrine of Jupiter is dimly illuminated with slowly burning candles. On the altar lies remnants of sacrificed lamb. You slowly rise from the floor, wiping sweaty forehead. Through the window you can see rising sun and quietly mutter prayer to Sol. From centuries your family has secretly worshiped Roman pantheon, fearing that pope will somehow get to know. But now you don't feel fear, you are strangely calm instead. You know what your purpose is now- no more secrets, no more fear. It's time for rightful rulers of Rome to regain their heritage. Luckily last years have been good for you. You've managed to convince [name of second Papal vassal] to become your friend... And your brother in faith. Your purpose is simple- take control of Eternal City, restore its former glory... And rebuild Roman Empire. You don't yet know how you are going to defeat impostor- Holy Roman Emperor, but you know that knowledge will come with time. Right now- its time to take Rome!
Decision: Roman eagle shall fly again!
Effects: Main character gets "Jupiters avatar" trait (+10 to all skills, immortality- can't be killed in battle, bonus to conversion, bonus in battles, can't get any bad trait), ambitious and diligent trait. Characters culture and religion changes to Roman. Ruler gets claim on Rome. Jupiter chosens holy order is formed. In holding new building is built "Legion barracks" +1000 to heavy infantry, +200 to light infantry, +300 to archers, +500 to light cavalry (after conquest of Rome it should be relocated there). Instantly war is declared upon pope.