Buy the Lumia 1520. Let me give you my address so you'll know where to ship......
Seriously, though, there are two schools of thought on this one.
1. You like to be budget minded, and the best time to get a flagship is when a new one is about to come out, and you can usually pick up the previous flagship pretty cheap and get more bang for your buck.
2. You want the best hardware available, and have the money to spend, in which case it is best to wait until you see what they come out with. If the new Lumia Flagship is $3000 dollars because it has a holographic display that hovers above the screen, then you'll have to decide on whether or not to take out an extra mortgage on your home. I doubt it would be that, but you never know what kind of features the new flagship is going to have until it comes out.
Microsoft hinted at new hardware that other Lumias simply do not have available and that it was going to be awesome. If seeing it once it comes out will make your burn with envy, then I suggest you do one of two things in this case:
1. Get over it.
2. Patience. Wait for it. Snap it up when it is available. Maybe they're coming out with the 1750.