I just have some ideas and I think others may as well so here's a thread for it.

Races;
I think if you could make your own or update the Lizardmen race (and include it in F.U.) that would be really nice! It's a cute race and hasn't been shown love in some time. Issues I have encounted is the inability to have my people have a preference for that race in terms of love.

A variety of dnd races would fit into Fantasy Unleashed like the Aarakocra, Kenku, & Owlin (bird-people). As well as Tabaxi (cat-like), Leonin (lions), and Tiefling.

Mythological beings like Satyrs, and Gargoyles would be cool to!

Monsters;
creatures like Draugr, Dryads, and Hapies. There's also jackalope, peryton, and cockatrice. These are all a good variety of hostile and passive - I think most could be tamable.

Biomes;
You can make a whole seprate mod for just real-world biomes like cherry-blossom forest or redwood forests. Jungles. Some ideas for more fastasy-esk ones tho;

1# A lightly-forested biome that has trees in various shades of light-blue, pale-purple, and soft-greens accompanied by mushrooms almost as large as the trees. Flowers a scattered in the forest in shades of pink and white, and the water is a deep purple tone. The grass is a blue-green color.

2# A plains-like region covered in flowers and crystals. The few trees that exist have flowers themselves (or just giant flowers for trees.) The stones have moss on them (or are just green), and this biome has bees- Including bees that produce special honey (like curing wounds, changing traits, shuffling romantic tastes).

I don't have much else to add currently but I might later. Also I do not expect anything to make into a mod just wanted to put ideas out there! I would love to talk to others to about their ideas!