I feel like World Runes replace the concept of a Nexus in the scope of this new canon as incredibly powerful fonts of magic, though I could see Nexi as being their own unique kind of font, formed deep within the earth and uncovered by humans ages ago. Given how you can apparently crystalize starlight crystals might be the coalescing of celestial energies, and since just about everything in the cosmos would've been made because of such energies they can collect and manifest physically on worlds.
I feel like the Godwillow, the Oasis of the Dawn and the sacred pools of the Blessed Isles all share some major connection such that I'm gonna be paying them extra special attention in the future. In contrast to any kind of arcane or celestial energies or whatever, these natural fonts are seemingly linked in to life energies and thus I'd imagine it's sort of linked in with Nagakabouros, but how I dunno really. Potentially the Godwillow and these pools were what allowed life to emerge on Runeterra in the first place, or perhaps accelerated its development at some point.