Yes tweaks will need to be made. I mean, Kate's dress was McQueen, but the House was known for its dramatic clothing and Kate pared a gown from that.
Meghan has a lot of natural charisma and charm and can pull off a sleek but dramatic gown with tweaks from the likes of R&R who are compared to old school McQueen in many circles.
Some examples of R&R that can work with tweaks. Meghan mentions Elie Saab as a favorite and these gowns have similar details to ES.
--https://ralphandrusso.com/couture/spring-summer-2017/lookbook?look=55
--https://ralphandrusso.com/couture/autumn-winter-2017-2018/lookbook?look=51
--https://ralphandrusso.com/couture/spring-summer-2016/lookbook?look=44
--https://ralphandrusso.com/couture/spring-summer-2016/lookbook?look=46