It is unusual nowadays for airlines to have an open return ticket, like what you see on some railway companies.

If I am you I would buy a return ticket which is dated as far as the calendar can go then change the travel date from that point. This would give me +/-1 year to play with dates, giving me more flexibility.

This would however mean needing to buy the most expensive fare bucket, such as Y for economy, W for premium economy, J, for business and F for First.

By buying these tickets you will be exempted from paying a change fee and difference in fare (as you are holding the most expensive one anyway).

Note that there would be a penalty for no show.