I watch a lot of HGTV and their shows about house hunting, first time buyers, home renovations and so on. It's probably my favorite cable channel.
I have learned by watching so many of the shows that outdated bathrooms can be a big deal breaker for potential buyers. And it's not so much that they expect complete luxuriousness. They just want newer, more modern bathrooms. They don't want to deal with the plumbing and all that goes with it.
So my suggestion would be when renovating a flip, take an extra look into the bathroom and see what can be improved there.
Make sure that it has a more modern look to it and is functional.