My wife and I do this sometimes with our regular trips to Tokyo -- I would suggest that you choose a date way out in the future close to a time when you might need the return leg -- and then you can change it later when you know the actual dates. Also - do not assume the return leg is half the value of the ticket -- it's usually less, sometimes much less...

If you never intend to come back -- then just buy the one way.... As the others point out it is critical to check the terms and conditions of the ticket -- some tickets may require the original route be flown, but others may offer some flexibility and even allow a return from another city -- typically within the same region.