All the speculation among Christians about if the Antichrist might claim to be the Mahdi of Islamic Eschatology barely acknowledges the differences between the Sunni and Shiite views of the Mahdi. But when it does in general the focus is on the Shia version because there are more details in their traditions to over analyze. For example, it's the Shiites who call him Imam, to the Sunnis he is a Caliph. I've tried to be an exception in my past speculations, but have hardly gone too deep into it.
The interesting thing I've discovered recently is, it's not just Christians who think the Imam Mahdi could be the Antichrist, there is a Sunni Website saying the Dajjal is actually the Shiite Imam Mahdi. And their reasoning is very interesting given my own speculations on a number of subjects. But I should of course say I doubt all or even most Sunnis would agree with everything on that site, it engages in a lot of weirdness similar to what I'm used to seeing from the most off the deep end Christian websites.
It kind of starts with how the Shiite view makes Imam Mahdi kind of quasi Divine, even giving him a Pre-existence. While the Sunnis see the Mahdi as not even a Prophet.
Another claim of this website is that Shiites don't always admit to being Shiites, and so ideas it's skeptical of in traditional Sunni sources they suspect could have been planted by Shiites. Since for most of Islamic history the Sunnis have been in control and haven't always allowed Freedom of Religion, one can't blame Shiites for having a history of hiding.
Today most Shiites live in Iran and Iraq. Geographically that happens to be the lands associated with the Dajjal in the oldest Muslim eschatological traditions.
The Dajjal was originally expected to be a Jewish Messianic claimant. This website points out many things that seem oddly Jewish to them about the Shia traditions. Like Traditions that the Imam Mahdi will use the Jewish Name of God (I've read other Muslim websites agreeing with conspiracy theories that YHWH is actually a pagan god), and rule in the manner of David and Solomon. And that specifically say the Imam Mahdi will have Jewish followers.
And many Jews have lived in the Irag/Iran region, going back to the exile and the Exilarchs. Also early Caliphs resettled the Jews of Khyabar in Iraq. I meanwhile could add my arguments that the Lost Tribes settled East of the Euphrates, and that the Persians and Iranians partly descend from Naphtali and the Eranites. In the context of all that, seeing the Shiite Imam Mahdi and the Rabbinic Messiah Ben-Joseph as possibly the same personage could fit pretty well. And that fits how Shiite traditions often treated Ali as the Joshua to Muhammad's Moses.
They are suspicious of prophecies that say he'll discover the Original Torah Scroll and Gospel, and rule the Jews and Christians according to their own laws. And also re-discover the original Koran as well. I've commented on the Mahdi's expected archaeological exploits before.
They are also suspicious of the Sufyani tradition, an enemy the Mahdi is said to defeat, who will rule parts of Syria and Iraq and will descend from Sufyan. When the split between the Sunni and Shia happened the Sunni followed a son of Sufyan, so it could be possible to see Sufyani as a code for a Sunni leader (maybe even Sunni Mahdi claimant) from a Shia POV. Today ISL/ISIS and it's desire to set up a Sunni Caliphate in Syria and Iraq could be seen as fitting the Sufyani description from a Shiite POV.
They also object to Shiite traditions that seem anti-Arab and even Anti-Mecca. If you're thinking "how could any version of a Faith founded by Arabs be Anti-Arab" just look at all the Anti-Semitism in Christian history.
The Koran was written before the split happened obviously. Thing is the word Shia was originally a word for Sect, so this site takes advantage of some Koran and early Haddith usage of that word, including references to the Dajjal having a Shia(Sect).
Thing about all this is, my emphasis on the overlooked fact that the Beast is actually against Babylon, and that his Seat is West of the Euphrates. Tells me that an Imam Mahdi claimant is far more likely to be a Decoy Antichrist then the real one, and perhaps one of the Kings of the East, or the Terrible of the Nations.
Does that make a Sunni Mahdi claimant the real one? Maybe, maybe not, there could be multiple Decoys.
And I still have my skepticism the Antichrist will be Muslim or Pagan at all.