Royal wedding dresses need to be a bit over the top, or else they look wrong to the cameras in the Very Grand setting. Prince William will presumably be in his blues, and all around will be gold braid, red robes and ermine.
So simple strapless/drape (like her usual style) won't cut it. She''ll need to glam up. Ivory lace would be a good choice, as you could have a stunning, close fitting dress with high neck and narrow sleeves, more volume at the back, and still attach a veil and yards of train.
Grown up bridesmaids could wear a simpler version, though I'm not sure what you'd put littlies and pageboys in.