For the CB we need to take these things into consideration:
- who all is able to expel jews? AFAIK all rulers that a) are not of jewish religion b) have lent money and are in debt. could be the emperor of the HRE, could be a 1-county count, who is a vassal of the HRE. could be someone whose realm is far away, in a hard to reach place, in the middle of enemy territory etc.
- who is supposed to get the CB? all jews? only the top rulers, those that have no liege? only those that live inside de jure israel or anywhere in the world?
- what is the CBs effect? the CK2 CBs define their target up front so its not possible to declare a "general" war then take the first you happen to occupy, the target needs to be defined clearly, the whole AI war strategy depends on that and it took a long time to get the AI to actually go for the war goals not some random provinces. options are, war for 1 county, 1 duchy, 1 kingdom. generally, CBs have a kind of ranking among power gamers, the 1 kingdom CBs are the best, the 1 county ones the worst, the ones that remove all nobles from the target are best, the ones that keep them and make them your vassals, the worst

other things to consider:
- how often does this happen, how often do jews actually get expelled, in an average game? where all does this happen, mostly in christian world? islamic world? everywhere? i wouldnt know any of this if someone asked me, not before measuring it somehow
- how many of these CBs would i have if i got them every time a ruler expels? how many would we want to give the player, as the mod creators? do we want them to constantly have a CB on 40-60 different rulers all over the world, or once in a lifetime?
- how would this change unbalance the game? will it be like a super religion afterwards, are there going to be power gamer strategies? play Byzantine, switch to judaism, get free CBs on the whole world and when the mongols come in 1200 they will find a purple jewish eur-afro-asi-india?

You can only expel Jews at King-level or higher. I don't think you need to have borrowed money; I do know only Christians can borrow money from Jews.
Maybe, to make it less OP, you can say only Empire-level Jewish rulers get the CB. Even if you don't, it should still be only at least king-level Jewish rulers. Maybe just Israel?
Maybe it wouldn't be a conquer land CB; that would remove the worrying about getting OP, and then we have the event-spawned troops.
I don't know how often the AI expels Jews; I just asked it on Reddit.
EDIT: I got the following answer on Reddit:

The AI tends to expel them only if they are in a massive gold deficit so probably whenever a huge war is waged or smth. And I found that Sunni Muslims expel them more.

That's only what that poster got, though. I'm not sure Sunni Muslims expel the Jews more in general.
SECOND EDIT:
I got another reply:

Germans do it pretty often

Yes well, if its random anonymous people from the internet then they must be 100% right, no? I still say, without measuring it properly, we will not really know, and measuring shouldnt be too hard, just need to add a log statement to the expel decision, run a game in observe mode, then write a small script that collects the log statements and analyzes them.

I did my first successful jewish game on ironman last weekend, and i have to say, if one knows what he is doing, jews are pretty much OP already since they already have a holy war CB on pretty much everyone else. I have actually found another problem that we could address in this mod, but as far as CBs go, the only thing i didnt like is that i was not able to declare holy war on christians if i had to reach them by sea. I can holy war christians when they border me on land, i can holy war muslims over land and over sea, but cannot declare any war on christians except "extort tribute" if i have to get there by ships. Thats something we should definitely change.

So maybe lower the times you can declare a holy war and replace it with the "Expelled Jews" CB.

Here's an idea:
Holy War CBs are only allowed for land in the de jure Kingdom of Jerusalem, holy sites, and duchies bordering the de jure land of your primary title (so, if you're King of Israel, you can Holy War provinces bordering the Kingdom of Jerusalem, but if you have an empire, then all duchies bordering the empire can be Holy Warred).
Then you can add the Expelled Jews CB in without overpowering Jews, since the Holy War CBs are vastly decreased.

I still dont understand, what is this supposed to fix? Is there a problem with CBs when playing a jewish character? Who would want to give up the regular holy war CB and get a very castrated crippled circumsized "holy war only inside k_israel" CB? Adding flavor does not necessarily mean, to make life harder or next to impossible for the player. Making things harder and more difficult is not complicated, i can come up with 100 ideas right away how to make the game harder for everyone.