You can do that. since one-way tickets are (almost) always more expensive. Booking a flexible return ticket will also be more expensive than the single ticket version, but you should look into it, since it should be cheaper than the double-return tickets in most cases.
But you better make sure that you know where and how to buy a ticket in Hong Kong and how much it will cost you AND you need to make sure which countries require you to have a home ticket before they let you in.
You will have to show return tickets on some immigration checkpoints or even airport check-ins. If you combine several stations and not have a valid return ticket, they will not let you travel.
Example: You fly to Hong Kong and stay there past the date of your intended ticket back home. You travel to Japan and then back to Hong Kong with the intention to buy a ticket back home in Hong Kong once you are there. At the check-in in Japan back to Hong Kong they will give you trouble already because you do not have a resident visa and you do not have an onwards trip booked.
Due to this issue, you might be better off to buy a flexible ticket where you can change the flight back from the beginning. Unless you can determine from the beginning which countries you go to, where you will buy what ticket and where you need an ticket back home or one for an onwards flight, you will have difficulties planning the trip without running into problems.
Or you book the flight back way past the intended return date - but close enough so that you are not indicating to an immigration officer that you will overstay your visa requirements. Then you just buy a new ticket to get back home instead of using your old one.
** Some more info regarding flexible tickets **
You have to know that for each flight, there are several different tickets with different prices - even if you buy at the same time from the same airline. The price depends on the flexibility of changing the ticket. If you want the cheapest ticket, you can get those, but changes can be VERY Expensive, up to the price of a new ticket. If you want to be more flexible, you will need to pay more for the ticket, but it you will be able to change it for a smaller fee.
I can only recommend you to call a travel agent to help you understand your ticket options. Since most online ticket sales are geared to sell cheap tickets, those often cannot be changed.