Surface: Beefalos. They're an essential means of transportation on the surface, and there are a variety of different tendencies a Beefalo could possess. Now, this variety is great and appeals to different play-styles, however, I think adding a variety of tameable mobs would be even more appealing. Each one could have individual benefits and drawbacks: ridable Koalefants could have an increased speed, maybe an ability utilizing their trunks to stun nearby enemies (similar to a Beefalo horn, but the Koalefant lets out a call and the sound disorients surrounding mobs) with the tradeoff being lesser health than the Beefalo. Also, a tameable Ewecus could be cool too, it would have ranged snot attacks but a slower speed. As for new utility for tamed mobs, maybe we could get a new slot for them. I'm proposing a new slot that could be used for extra storage (bags or pouches strapped to the side of the tamed companion) or armor.
Caves: I would appreciate some form of ridable mob that resides in the caves. Maybe a salamander, bear, cave cricket, centipede or beetle.
These are things I thought might improve the diversity of the game, I would like to hear some ideas you may have about this. Thank you for reading, folks.