These wouldn’t necessarily impact gameplay, but would be nice for immersion and refreshing visuals
Changing seasons, day and night; basically some variation during a match. optional: bodies of water freeze in the winter, opening up new places to attack through. Occasional storm events, fog?
When you group units together, it creates an Army that you can customize visually (i.e. unique coat of arms, designs, etc). It then gets flag-bearers with customizable designs/textures (hint: modding!). Perhaps make the armies nameable
Leave corpses, siege/artillery wreckage, flags from battles in landscape for as long as allowable with current-gen computers
*HUGE herds of wildlife (perhaps instead of hunting single animal, you can hunt a herd)