I'll probably have to wait until the lockdown is over, so just dreaming of ideas 
So the idea is... a mix of colours, informal, no structure, no order? Well, obviously taller perennials at the back?
Any ideas? Is it just a case of packing them all in? 
I'm thinking delphiniums (although I read that they only last 2-3 years)?, foxgloves, Lavender, Roses.