I’ve asked this question to quite a few people in person and so far haven’t seen a good answer...but but I believe one should exist, so here goes!
Ok, we all know how to (roughly) prove Fermat’s Last Theorem:
Each solution to a^p + b^p = c^p$a^p + b^p = c^p$ gives a Frey curve, an elliptic curve that is cleverly designed to encode the difference between trivial/non-trivial solutions geometrically (i.e. singular/non-singular Frey curve) and has a very special conductor.
One then invokes modularity in the non-trivial solution case to produce a modular form of a special level...which which then is proved to not exist by level lowering arguments. Hence FLT is proved.
One of the current goals for many is to prove paramodularity, which in a nutshell should see certain abelian surfaces correspond to certain genus 2 Siegel modular forms of paramodular level.
The conjecture bas been made precise, for example see Conjecture 1.1.1 of:
https://math.dartmouth.edu/~jvoight/articles/faltserre-paramodular-073018.pdfthis paper.
But anyway the details don’t matter too much. My question is the following naive one: if modularity is to FLT then paramodularity is to what?
Now I am not claiming that paramodularity is useless without Diophantine applications. However I feel it would be nice to know if it can be used to show there is a certain Diophantine equation (or even a family) that has no non-trivial solutions.
Perhaps there is a simple way to generalise the Frey curve construction to give an abelian surface which encodes the triviality/non-triviality of solutions for some Diophantine problem geometrically?