I can imagine AGI just fine. Doesn't get me one tiny little bit closer to being able to make one. There are several ways in which one could go about such a development, all we have for now is an existence proof and none of the paths pointed out so far have been viable. Whether John will come up with novel path is not really an imagination issue but one of very deep understanding of the problem space, what has been tried so far, why it did not work and then to come up with something that either fell through the cracks as non-viable and then to recycle it in a way that it is viable (the current neural net applications are like that) or an entirely novel approach. The latter will likely come from an outsider but it would be an extremely lucky shot to hit something workable; the former may be a possibility worth investigating.

The reason why the latter has some chance is there are sometimes approaches tried early on in a field that can't succeed because something else needs to be invented first; or the computing power required is prohibitively expensive. Carmack's skills and ability to absorb knowledge might help to spot such an opportunity.