I've mostly confined my points on it to Facebook, but I'll put this here, for people who want to know my take:
With the caveat that I literally got up half an hour ago and so have not seen any coverage since 7am, UK time...
ISIS is also my chief suspect. The methodology could go either way, Al-Qaeda or ISIS, with Beslan, Mumbai etc. The gratuitous nature of the violence also suggests ISIS over AQ, the latter being slightly more stringent with targeting (see Charlie Hebdo, frequent AQ complaints about civilian Muslims being collateral damage etc).
According to some reports, just before I went to bed, one of the gunmen shouted "this is the fault of Hollande, he does not have to intervene in Syria" before he killed himself. I suggested, back in 2014, that if overt intervention in Syria looked to be on the cards, the strategic calculus would change for ISIS, that while they would encourage and agitate for attacks in the west, they would prefer to focus on their more immediate enemies except in the case of potential attack. In the long term, I think ISIS would always be looking at attacking the west, it has to wrest the crown from Zawahiri somehow, and beyond that, their overall ideology paints the west as enemies of Islam.
Another thing not to discount is their awareness of the refugee crisis in Europe, and how an attack could complicate the political response there or even work to inflame tensions. ISIS are remarkably media savvy, and I'd be very surprised if they weren't aware of how such an attack would, almost inevitably, be followed by calls to refuse refugees, crack down on Musims generally. That then plays directly into their propaganda. It also sends a message to the refugees - no matter how far you run, we can reach you.
Another thing, that was mentioned in the Vice article but that I neglected to mention above. ISIS has been put under quite a bit of military pressure lately. I suspect it's more of a short-term issue than the death knell of the group, given their ability to bounce back.
That said, a large attack like this...well, it's advertising. Get more men and women joining the jihad. Convince the potential donors in the Gulf states that ISIS is leading the Islamic revolution, and so deserves their donations. An influx of recruits and money could be just what they need to turn things around in those areas where they've been taking a beating recently.