Okay well I have some ideas for races.
Giants(under another name if possible) - sapient race that share characteristic with humans but are way larger. They were once a mighty civilisation that brought down mountains and constructed immense cities atop them, but after a plague they are very few and are organised in small clans, very rarely under an actual kingdom.
Guardians - a race of human-plant halflings that use powerful magic to protect their ancient forests from human eyes. After many centuries of humans trespasing their forest they became very hostile and keep some forests under complete lockdown, that armies of men cannot surpass. Mysterious and pl*ps
Harpies(this is a really cool idea imo, going back to the history of this custom world) - a sapient race of human-bird halflings that once ruled over all humans. Eventually a war came since humans wanted freedom, and they eventually killed all male harpies. The females were allowed to live in peace, and due to their knowledge they generally take high positions in the hierarchy. The harpies cannot breed males, but insted more harpies. Without the humans they would die off.
And another idea is to have some "old gods" that both he harpies and the giants worship and are like the daedra(expecting blood sacrifices and such) humans involved in those kinds of rituals are generally very powerful magical users. Through those sacrifices they become almost unstopable(would be a direct reason of why my kingdom would be hating most mages)