Well how many independent muslim republics do you know ? Even Cordoba was a republic under a feudal lord and his goodwill as far as i understand.
Thats not a republic, rather wishfull thinking and it lasted exactly how long ? 60 years or something?
Well, the Rashidun Caliphate itself was a republic early on, with its leaders being elected (indeed, this is something of a catalyst for the Sunni/Shiite split). More current to Crusader King's time is the works of al-Farabi, who advocated for an ideal Islamic state with a democratic government. There's also the tradition of Shura (consultation), which can be (and has been) interpreted as support for democratic/republican ideals.
It's true there isn't a lot of others (though what constitutes a "republic" is always contentious), but that's why Cordoba is so interesting - a spontaneous development of a republic state in the Islamic world, when that hadn't been the norm since the Rashidun Caliphate. While it did fall soon after game start, that is avoidable (it was due to a civil war between the original founder's grandsons, leading them first to start trying to strip the democratic powers from the republic to bolster themselves and then one of them to invite the emir of Seville's troops in to help him, which predictably ended badly for Cordoba's independence). Merchant-wise, however, the Islamic world was booming, and the Karimi merchants controlled much of the entire world's trade at this time (and pre-crusades, had notable Jews and Copts amongst them).
It'd take several tweaks on the Italian system to represent this adequately, but that's precisely why I find it disappointing the Republics DLC doesn't. While it'll undoubtedly be easy to mod to include Cordoba and any other Islamic republics, the system, flavour events, etc. are all going to be for Western European Catholic republics - and to be honest, primarily for Italian republics as Gotland was minor and the Hansa doesn't exist at start date. I didn't expect a full, in-depth treatment of Islamic commerce and assumed Venice and Genoa were always going to be the stars of the show, but some flavour and enhancement for Islamic merchants would have been a welcome addition, and paved the way for some goodies for non-Abrahamic republics when that DLC comes out. Now, obviously, if they are not playable and not meant to be playable, it seems very unlikely there'll be much if any thought into how non-Catholic republics should even work (ie, considering how it impacts Muslim decadence mechanics), much less flavour events et al.
I'm not upset at Paradox for doing it - it seems they came up with a system tailored pretty much to only represent a very limited number of maritime merchant republics, and therefore applied it only to them, which is understandable as far as it goes. I'm just disappointed that at present, that cuts out the possibility of meaningful gameplay mechanics for other republics. The only one of those five I'd even consider playing is Gotland, and it's easily the least historically justified inclusion, so that makes the DLC a lot more unattractive to me, unfortunately. Perhaps later down the track a more inclusive look at republics will happen too.