There are several mods being added for the Mini Season to see how well they perform in a massive multi player setting. They may or may not be included in a full season.
Odin Horse adds tameable horses that can attach a cart or be ridden.

Seasons adds four seasons.

A mod that adds several new boat options

Added dungeons for orres (i.e. copper, tin) across the biomes with configurable resets

There are a few other mods that being added and tested that may not be immediately noticable.
The following mods have been removed from the mod pack for Mini Season 3. They may return again in the future.