These could be complicated to add/definitely take time, but I'll share the ideas just in-case if they're possible

Animation blends-

It basically allows you to add strafing and add a smooth transition between animations based on speed.
For example, let's say you've made 8 animations, 4 walk animations, 4 run animations.
Each 4 animations have- forwards, backwards, left and right, the walk would be closer to the center and the run would be further out on the blend.

This would allow you to make your movement cycles "blend" together nicely using speed instead of walking/running being a sudden switch, and it could prevent moonwalking when walking backwards.
This could also be used for let's say, falling.
You could have a normal jump/fall animation, and the faster you go, the more you blend into another animation (like a normal into a panicked fall).

And let's say you can also make animations for when you go beyond the normal running speed like a sprint within the blend.

Extra Poses-

Taking blends into account, you could make more animations work with others.
Let's say you have a complicated walk/run cycle, and you crouch with it. It can sometimes act strange.
Extra poses like Crouch Walk, Crouch Run, Attack, Block, Land, Skip/Hop, Jump could be useful, and other animations could be added if needed, and maybe bound to a certain action you can do in-game.

Viewport Animations-

It's what it says, let's say you've finished making your model and animations, and your arm may be in a strange position, or something's off with movement.
You'd be able to change different things like the Default Position of your hand(s), Block animation, Attack animation, stuff like that for each arm.
Also maybe the ability to change the camera offset for different heights, and add some sway/movement to the character when you do certain things (like a custom camera bob).