  We show that two-dimensional fermions with dispersion $k^2$ or $k^4$ undergo
a first-order Stoner transition to a fully spin-polarized state despite that
the spin susceptibility diverges at the critical point. We extend our analysis
to systems with dispersion $k^{2\alpha}$ and spin and valley isospin and show
that there is a cascade of instabilities into fractional-metal states with some
electron bands fully depleted; narrow intermediate ranges of partially-depleted
bands exist for $\alpha<1$ or $\alpha > 2$. The susceptibility becomes large
near each transition. We discuss applications to biased bilayer and tri-layer
graphene and moir\'e systems.
