I too love mixing prints. Hard to get right, but so worth the trial and error. I'd start with an oversized plaid or stripes of varying sizes. Does not have to be blue and white. Might look better if it is not.

All of the sheets you posted are mini-prints, very pretty. A large scale floral or Jacobean print would work very well. A good way to think about mixing prints is to vary the scale--so one each small, medium and large scale print in similar colors work together nicely.

Here are some from Pottery Barn that would work with your sheets:

https://www.potterybarn.com/products/dahlia-floral-duvet-cover/?pkey=cduvet-covers-shams

https://www.potterybarn.com/products/lucia-floral-duvet-cover/?pkey=cduvet-covers-shams

https://www.potterybarn.com/products/bloom-floral-print-duvet-shams/?pkey=cduvet-covers-shams

https://www.potterybarn.com/products/jardin-toile-duvet/?pkey=cduvet-covers-shams

Also what else will be in the room? rug? curtains? These should be taken into account as well so you don't have clashing patterns.

I am not good at mixing prints and have no desire to do so . I do however find some mixes very pleasnat ot see but as mentioned it takes some skill. Each sheete set you show seems to have blue and white in it so I think a nice blue and white coverlet could be great with a nice darker blue blanket or throw maybe in a nice wide stripe We do however need to see the room so post some pics here in a comment DO NOT start another post.This might be a gutsy as I would do