Don't buy an around the world ticket. I can't recommend it. Long before we started our trip around the world in 2010 we discussed whether to buy one or not. But one of our main reasons why we didn't buy one, is that we wanted to be as flexible as possible.
Yes at the beginning we had a route what and which countries we want to visit, but we canceled it in the first two weeks. You don't know what happens on the road. You will meet so many people and they will tell you stories what you definitely have to see or have to do that it is impossible to plan these things. One of the main reasons for me is flexibility when I'm traveling. I don't want to plan what I want to do. You will change your plans anyway. I met so many people with a world travel ticket and 90% wouldn't buy one a second time.
2. One Way Ticket
That's what we bought and it was one of our best decisions at the beginning of our trip. We bought a one-way ticket to Halifax, Canada from Frankfurt for around 300€. Ha ha, I remember. We didn't know anything about Halifax in Canada. The only thing we knew was that it is a city on the east coast of Canada. We only had a couchsurfer for the first two days in Halifax. We hadn't any travel guide for Canada and didn't know how to get from the airport to the city. So we hitchhiked from the airport to the city. That was our first autostop in our lives and definitely not the last. Again, you have to be flexible, with hitchhiking even more.
Yes, an around the world ticket might seem to be the cheaper option, but I think it could end up to be much more expensive than to buy every ticket individually. You will change your mind on a long trip what you want to see and you will have to change your following flights. Anyway, in Central America and South America, you will be better off by taking a bus than to miss this beautiful landscape. In Asia, you can find very cheap flights without an around the world ticket.
An around the world ticket is only valid for one year. One year sounds like a long time, but I'm honest with you it's almost nothing for a trip around the world. You can spend a year in a country in South America and you'll not see the whole country. I mean if you go to a trip around the world, I'm sure you don´t want to see only the "highlights", you want to meet locals and spend some time in a country. This takes time. If you have an around the world ticket you have to worry about the time. After two weeks traveling we forgot which date it is. It's not important(except if you're hungry and it's Sunday and all the supermarkets in Canada are closed and you are almost starving to death...). To live day by day and no need to worry about the time is totally enjoyable.
And there is one more thing. I'm sure you can't stop traveling after a year if you have still enough money and even if not it is hard to stop.
Don't worry about to enter a country only with a one-way ticket. We didn't have any problems with that. Nobody asked us about it. Not even when we entered the USA by bus. But if that's the only thing that bothers you than I can give you the advice to buy a bus ticket to the next country. So you can prove that you travel further. That should be enough and it's much cheaper than to buy a flight that you will not take.
Yeah, I'm honest with you, you will not come back as the person you were when you started your trip. You will change even if you don't think so. Don't plan one year ahead. If you go traveling a long time you are looking for something. Maybe you don't know it yet, but you will find something. Just let it happen.