regarding physical limitations on lens sizes

not talking about front element diameter, because i do get that's pretty set in stone, but what about lens length? these so-called limitations today are not taking into account future advancements in materials science and optics.

what are these lenses made of? is it possible some ultra-high-index material will be developed that will be able to bend light much more severely, and you can have pancakes of most focal lengths?

what about other advancements people aren't considering?