This is an interesting idea. I've not actually tried growing wildflowers in pots before, but I am trying it this year, as an experiment. There is a wide range of wildflowers, and I'm only trying a tiny sample.
... You could try red clover ..
This is red clover, in a pot. Until I saw clover growing by itself, I didn't realise what the plant actually looked like, or how it might want to behave. It actually wants to send out runners, somewhat like a strawberry...
... and birds-foot trefoil too
And this is Bird's Foot Trefoil...
Foxgloves...
This is a Campion, actually growing in grass, but I think it would make a decent pot plant...
This one's called Salad Burnett, the flowers are a bit dull...
Scabious (a plant from last year), good for bees and butterflies...
I think wild marjoram would be good too. I'm raising this from seed, sowed a few weeks ago, so they are only tiny seedlings at the moment...