Coronation Gown

Butterick 6022 - perfect for your Coronation

The only new pattern I liked was this one, because you just don't see that many Coronation-worthy gowns.

I'd do the collar/capelet in ermine for that. Heavy silk of course. Needs a bit of work in the embellishment department, but my loyal subjects would take care of that. My other quibble is that the train needs to be longer. (Westminster has a very long aisle), but that is easily adjusted.