How would a completely different alien civilization describe this universe? Is the answer we have no idea? They are completely alien. Is there an external reality that can possibly be described in ways that we have no idea of? Exactly like a theory we have no idea what it looks like. There will be new theories that will supplant quantum mechanics, general relativity and our standard model of particles and forces. All very successful theories but we know fundamentally they have problems and limits where they are not applicable. As some examples: what's inside a black hole? , or the Planck scale. The three mentioned theories will become effective theories in which they are limits to what the new theories will tell us about our universe. We have no idea what they will be like. Fundamental physics is stuck and has been stuck are awhile. Stuck probably isn't the right word. Of course physicists have been searching for this (these?) new theories. Einstein spent the later 30 years of his life searching for a unified theory. The problem was that he wasn't including quantum mechanics. With the development of quantum field theories we saw in the 60's the Higgs mechanism and the electroweak unification. Both leading to Noble prizes. In the 70's and 80's we saw the development of quantum chromodyncmics (QCD) to describe the strong nuclear force that led to at least one Noble prize. Then various "Grand Unification Theories (GUTs) to combine the electroweak theory with QCD. These GUTs models never quite panned out with some predicting magnetic monopoles that were never observed and some predicting proton decay and lower bounds were set for those searches since no proton decay has been observedin some quite elaborate experiments such as Super Kamiokande.
People have been working on quantum gravity from at least the late 40's. Feynman taught a graduate level class on quantum gravity in the 60's, here's a book form.of that class. John Wheeler had also been interested in quantum gravity and is this possibly a reason why he had two of his students Feynman and Hugh Everitt III develop new ways of interpretation and using quantum mechanics for their dissertations? String theory was a possible way to unite quantum mechanics and general relativity. But so far that theory is plagued with giving the string theory landscape .
Maybe some new form of mathematics needs to be developed. Newton invented calculus to describe his ideas in physics. Einstein used Riemann's form of geometry to explain his theory of general relativity. Some string theories use Calabi-Yau space to describe extra dimensions. String theorist Ed Witten even won a Fields Medal in mathematics for his work So who knows maybe some new form of mathematics needs to be developed.
Or maybe something completely different is needed. The end of reductionism? Some completely new unheard of paradigm shift?