I had this idea to maximize the amount of space in my small bathroom, to redo it and the kitchen in one go, put the kitchen sink next to the bathroom, and skip the bathroom sink. A quick internet search doesn't give me any examples of this being done, but I can't think of a reason not to do it. Sure, it's a little strange, and guests might think I'm crazy, but hand-washing is just a door away... Is there a reason not to do this, beyond "not normal", or resale value, because I've already considered this ? Are there examples of this being done anywhere ?
Ratchet Freak's suggestion is good, though they are always a little awkward. But do have a big benefit of being the only plumbing fixture you need to plumb.
Before committing to that, though, do note that they make really small sinks. A few examples from a quick Google image search:
Unless every one of your guests wipes the inside door knob down (and disposes of the cleaning material) each time, you have no guarantee of hygiene, assuming you do the same thing. Each time.
And, it's a kitchen. We humans have evolved to that point where we generally don't allow (or try not to allow) fecal bacteria to commingle with the stuff that we eat.
What if you need to clean up a little kid who's not quite potty trained? Would you go back and forth between sink and toilet? How about vomit in the bathroom? How would you clean that up without needing to rush back and forth? So many other use cases come to mind.
Yeah, I wouldn't recommend this.
The are toilets with a sink built into the tank, an image search of "toilet sink" will show you several designs.
These have the upside that you save a bit of water as the run-of of the sink will be used to help fill the tank after you flush.
I think you should always consider resale with remodeling projects. Without a sink it is no longer considered a half bath. A buyer would look at it as an expense and a "have to do" project. As a woman and former Realtor, put in a sink! There are plenty of space saving/smaller size sink options available. Sanitation, resale, convenience should be enough reasons to put in a sink.
Ignoring the hygiene implications, there's a social reason not to do this. We all know that hand-washing after visiting the toilet is advised, but many people don't bother. Particularly after a less... significant trip to the toilet.
Having a sink outside the toilet draws attention to the habits of others that perhaps you'd rather not know about.