We are planning to fly Istanbul to Sydney with Emirates.

We plan to fly out October 2016 and return September/October 2017.

Currently we can get a very affordable one way ticket (about 65% the cost of return ticket); or purchase a return ticket (coming back in April 2017).

If we purchase a flexible return ticket it appears we can modify the ticket but at a cost; and likely risk of having to pay the difference in air fare (if any)?

What's the cheapest option?
Is there any way to purchase a 12 month return tickets these days?

(We do not need to show evidence of return ticket.)