It really depends what you want, you need a wizard and apprentice. And then it’s up to you how the war and is built.
really the only thing that I would suggest is trying to stick as close to standard sizes.
not a hard rule, but too big or small can be a huge effect, but it also ads some fun so it’s a feels it out sorta thing.
so, beastigors and gors for a beastmen could be cool, if you want a beast group that isn’t the gnolls. Just need to work out wizzie and apprentice, and some shields and swords, daggers and such for specific wargear.
Gitz would be easy, but maybe expencive if you want specific one off minis
Sylvanath is doable as well, and lots of elf, chaos and undead can fit in with a little work.
it may depend a lot on your access to terrain, on a board with lots of buildings bigger and smaller units have less impact so you can go more wild. Only a terrain board that doesn’t have to much height, your big models will die, you will need to be paranoid of every direction, where a small mini like a goblin will be a terrifying foe. So if you want to go with more interesting group I would make sure you have good variance of terrain, tall and wide. With smaller scatter
Personally I have minis from everything when I play frostgrave, so plenty of Fantasy/Aos minis make it in at times.