Candelabra Primulas, Rodgersia (flowers a bit insignificant but great looking leaves) Minature Gunnera (same as Rodgersia but huge leaves) Equisetum tall architectual grass (this will spread via roots over years but can be planted in the ground in a pot) Iris.

Hope this helps.

Ligularia, Zantedeschia aethiopica 'Crowborough', Lobelia cardinalis, Lythrum salicaria. All these should be hardy.