As TSG said two of the options are to incorporate the transition, or hide it. It is hard to know which will work best in your space without knowing how big the crown will be and what style you have in mind, etc. Here are some examples of incorporating the window trim with the crown: [[(https://www.houzz.com/photos/traditional-bathroom-traditional-bathroom-new-york-phvw-vp~1034813) [Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Stamford Architects & Designers Sound Beach Partners LLC [[(https://www.houzz.com/photos/shingle-style-home-in-hanover-nh-victorian-bathroom-burlington-phvw-vp~106093) [Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Norwich Architects & Designers Smith & Vansant Architects PC [[(https://www.houzz.com/photos/chestnut-street-bathroom-traditional-bathroom-boston-phvw-vp~96002) [Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Boston Kitchen & Bath Designers Venegas and Company [[(https://www.houzz.com/photos/case-design-remodeling-inc-contemporary-bathroom-dc-metro-phvw-vp~87107) [Contemporary Bathroom[(https://www.houzz.com/photos/contemporary-bathroom-ideas-phbr1-bp~t_712~s_2103) by Bethesda Design-build Firms Case Design/Remodeling, Inc. [[(https://www.houzz.com/photos/weston-residence-traditional-bathroom-boston-phvw-vp~2583807) [Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Boston Architects & Designers Catalano Architects [[(https://www.houzz.com/photos/louisville-traditional-bathroom-atlanta-phvw-vp~116461) [Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Canton Furniture and Accessories The Furniture Guild Or transition with window treatments: [[(https://www.houzz.com/photos/fan-victorian-bathroom-los-angeles-phvw-vp~207997) [Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Los Angeles Interior Designers & Decorators Just Joh [[(https://www.houzz.com/photos/bathrooms-traditional-bathroom-phvw-vp~2678396) [Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Chester General Contractors ColeBuilt [[(https://www.houzz.com/photos/vacation-home-on-lake-michigan-traditional-bathroom-grand-rapids-phvw-vp~3286791) [Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Detroit Interior Designers & Decorators Urso Designs [[(https://www.houzz.com/photos/eclectic-bathroom-eclectic-bathroom-atlanta-phvw-vp~137747) [Eclectic Bathroom[(https://www.houzz.com/photos/eclectic-bathroom-ideas-phbr1-bp~t_712~s_2104) and this one, sort of in between: [[(https://www.houzz.com/photos/bluff-plantation-traditional-bathroom-atlanta-phvw-vp~58127) [Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Charleston Architects & Designers Frederick + Frederick Architects I hope this gives you some ideas!

Only other thing I can suggest is, if you know you want window treatments, you can have them box out the crown to leave space behind for the window treatment. I did this in my kitchen so the valance is on a tension rod and it gets tucked up and under the molding which sits about 3" from the wall. So the molding itself becomes the "valance". You can see the gap here, before the kitchen was finished. If you wanted to do that, you might want to make the box wider if you want side panels. [Traditional Bedroom[(https://www.houzz.com/photos/traditional-bedroom-ideas-phbr1-bp~t_715~s_2107) by Oak Park Photographers charlie mayer photography [Spaces[(https://www.houzz.com/photos/home-design-ideas-phbr0-bp~) by Dc Metro Home Builders National Association of Home Builders

If you did the crown in semi at this point (with the rest of trim being a flat finish), the crown is probably going to look a little out of place (in my modest opinion). I don't know how long ago you did the rest of your trim in the flat finish, but I've found that scrubbing a flat finish to be a pain in the neck. I paint my walls in what's called an eggshell (not color but finish) or satin finish, which is not as flat as flat finish, and my trim in semi-gloss.

Interesting, as I feel like the millwork is traditional and floor to ceiling drapes that are on each side of the window would be more appropriate to the style (not that one needs to be a slave to a style). I would mount rods outside of the millwork (on the wall) but have them extend out sufficient to clear the trim, such that the drapes, when pushed back will clear quite a bit (but not all) of the trim. I like my drapes to be able to open completely so I don't lose the light! Room is lovely!