First and foremost: Obviously, this is just an exercise in fantasy, because she will, with almost 100% certainty, go custom. But that doesn’t mean we can’t speculate — and look at pretty pictures of pretty wedding dresses!
And also: I think we should start laying our bets as to who is going to get the job. Obviously, it’s not going to be Alexander McQueen because Meghan is not going to want the comparisons to Kate if she can help it. (This is possibly a huge relief to Sarah Burton, because she nailed Kate’s dress and you want to go out on top as far as this particular challenge goes.) It probably won’t be Ralph & Russo, because they got the nod for her engagement photos. It seems HIGHLY likely the designer will be British (or maybe Canadian); my personal working theory is Erdem, although (despite what she said in her Vanity Fair interview), Meghan actually doesn’t wear them that often. On Twitter, one of you suggested Stella McCartney, and pointed out that Stella’s mother was an American, and I like the symbolism there (although putting Stella in charge of this makes me a wee bit nervous). I personally like the idea of giving Victoria Beckham the nod, mostly because I want the best for Posh and not because I actually think this is in her wheelhouse. Meghan’s probably talking to Jenny Packham (who is also probably too well-associated with Kate), the folks at Burberry, Vivienne Westwood, and all the really classic, blue-ribbon British designers. Meghan is famously very close with Misha Nonoo (who designed that great white shirt she wore at the Invictus Games), but Nonoo does mostly daywear and I just don’t think she’s going to get this one, a perhaps awkward truth between friends.
For reference, the last royal wedding of a couple about Harry and Meghan’s age and rank at St George’s was Sophie and Edward’s wedding, and Sophie definitely didn’t go for a big poufy frock. (More recently, of course, Chaz and Cams made it legal there, but I do not expect Meghan to pop out in a modified coatdress, even if Camilla did look very fetching; Peter Phillips and his wife Autumn also tied the knot at St George’s in 2008. ) That doesn’t necessarily mean anything, but we certainly want to keep history in mind, and it’s not a HUGE venue, so a Diana-style forty foot train would look out of proportion. And speaking of history, here’s the dress she wore for her first wedding — which, obviously, is a whole different ball of wax, but I think we can assume she might want to go in a different direction.