Siegel modular forms of degree three and the cohomology of local systems

We give an explicit conjectural formula for the motivic Euler characteristic
of an arbitrary symplectic local system on the moduli space A3 of principally
polarized abelian threefolds. The main term of the formula is a conjectural motive
of Siegel modular forms of a certain type; the remaining terms admit a surprisingly
simple description in terms of the motivic Euler characteristics for lower genera. The
conjecture is based on extensive counts of curves of genus three and abelian threefolds
over finite fields. It provides a lot of new information about vector-valued Siegel modular
forms of degree three, such as dimension formulas and traces of Hecke operators.
We also use it to predict several lifts from genus 1 to genus 3, as well as lifts from G2
and new congruences of Harder type.