I added -50 to base reluctance (+20 in vanilla) and +0.5 per player opinion for success. There also some options originally available like +50 for defensive call, +5 for common language, +10/+30 for court shared language, +50 for diarchs etc. So, currently you need to max your relationships and gain some extra like common language for the offensive call and it's possible you will stay alone in defense.

Other interesting thing is: you gain -20 to opinion for 10 years for every call, like asked gold from Pope. So, check your allies often like you do it with your car, or maybe hire troops like it's in Iberia.

Same for dynastic interactions (calls) but you have -45 base reluctance and no profits for common languages.

If you have something to propose - write here. Didn't check much but hooks looks to be fine for calls in vanilla as well.