What if in gen 9 the new gameplay change is that moves now have dual types?
For example, making Muddy Water a water/ground type move.

If you attacked a fire type, it'd do x4 damage.
If you attacked a fire/flying, it'd do x2 damage because of ground immunity but water weakness.
If you attacked a grass type, it'd do x0.25 because double resistance.

Of course, not every move would become dual type, but some of them would. If a Pokemon is immune to one type, it will only consider the other type

>fire, ice, thunder punch get dual fighting type
>fire, ice, thunder, poison fang get dual dark type
etc