There is nothing more annoying than people starting a post by implying they know their stuffs only to end up with the most basic, ignorant conclusions.
Those you call extremists are not the extremists of Shia or Sunna,they are their own thing, ISIS is for instance is just a modern freak that combine aspects from the Khawarij and the Assassins with modernist/post-modern elements.
Keep in mind that Sunna weren’t formed as a “sect”, they were the anti-sect guys as such they were always defined as the opposite to whatever new sect was rising untill around the forth-fifth century where the madrassa system started spreading, and who was the very first sect to immerge? The Khawarij, so imagine how annoying it is to confuse sunnists with their very first enemy, Shai weren’t even THAT relevant for many centuries and the prime ideological opponents for sunnist were the Mu’utazillies and similair sects for most of the early centuries.
Wahabism is literally named after its founder Ibn abd al-wahhab…it was basically the guy’s attempt to “fix” Hanbalism(one of the four surviving Sunnist theology/law schools) so in another word abridged Sunnism, which is why every expert agree on Wahabism being a modern movement as it is a reaction to its envirement first and formost especially its war on superstition, tribalism, nationalism, ultra-suffism and blind imitation of past scholars …the biggest problems however were that Ibn Abd-Al Wahhab himeself was as much of a politician as he was a scholar and that he wasn’t special as a scholar and yet become a figurehead that influencial, a good counter exemple is Shah Waliullah Dehlawi someone that is far more competent and also did his best to ‘‘fix’’ things and was as a result influencial on almost all reform mouvements in the Indian subcontinent and SEA.
so yes Wahabism does display hints of what will eventually come with Isis but the difference is that its problems are rooted in deep misunderstanding and bad practice and proficiency of both theology and especially jurisprudence, while for the likes of Isis those mistakes are their very basis upon which they build everything, ex: a Wahabist may call X muslim an infidel based on superficial reading of either that muslim’s practice or the scripture, while for Isis your Islam was always in doubt until you prove otherwise (by thouroughly anouncing allegience duh), a good show of this is how Wahabist had no problem fighting alongside Afghans vs Soviets despite them being mostly Hanafi(another classical sunni law school, Taliban are to Hanafism what Wahabism is to Hanbalism, kind of), while Isis just bomb everyone and everything.
their Khawarij similarity are way too striking really, the reason I also added the Assassin is more because of the way the group is structured(like a cult) and also to deal with the parts where they are different from Khawarij for instance one consistent patern with Isis is their members never showing any sign of religiousity prior to joining them or up until they partake in a attak(like the recent ones getting caught in a bar) when the Khawarij were leading priest/monk like life and killing was just another part of their day.
as for Ibn Taymiyya, sadly I don’t think In can clarify it in one post and the only accessible non academic piece on him is hot garbage(the video by Lets Talk Religion), just understand that you are talking about someone that simultaneously inspired every side of reform mouvenments including even Shia and non-islamist ones, people desperately needed an authority to lean on and he was perfect in many ways…yes he was a Hanbali scholar that can also be counted as independent and yes he is definitely a proto-Salafi but we are in a situation that the groups usually attached to him(Wahabism and Salafism) can’t even read any of his actualy long books, as they lack the tools to do so and will need to first study things they consider innovations and leading to herisy so like everyone else they take whatever they like and pretends the rest doesn’t exist. they are not the only ones either:Tabataba’i for instance was ready to close his eyes on Ibn Taymiyya takes on Shia and celebrate his critics on classical Logic, some secularist would also lean on his reverence of ‘‘Ijtihad’’ that is not being tied to immitation of previous scholar to pass rullings that will make him roll in his grave in agony and so on…you are talking about a character in the magnitude of people like Kant, Marx, Descarte…except due to reasons he was pretty much forgotten not even a century after his death and half a millinium later he was called upon by all, of course it will lead to chaos; it was only this past decade while living in what we call “post-salafism” that we started to trully gasp his thought as it is but the damage is done and he will continue to be viewed the ideological inspiration of extremist for many long years the same way poor Al-Ghazali is still somehow responsible for the fall of science in islamic history (lol), however I think some current parties of Salafists are kind of a close enough representation, however I don’t know if they exist in non-arabic speaking countries. just keep in mind that Ibn Taymiyya supposed followers varries from Isis to probably the most active Arab intellectual in recent years, whom started a big movement of translating major works both oriantalists and otherwise(like the Handbooks series issued by many elite universities) and were responsible the most for bringing in and populating Heidegger and Wittgenstein.so one must be careful what to attribute to him.