"I would like to get a one-way-ticket to Bangkok, as I will stay there
for a while. I have seen that tickets for a roundtrip are cheaper than
a one-way-ticket, so I thought about getting a ticket for a roundtrip
and just not to use the return-ticket. What do you think, could that be
an appropriate solution? Or is there any other possibility to get a
one-way-ticket that is cheapter than a roundtrip-ticket?"

When you're talking about flights between continents, one-ways are
almost always more expensive than round-trips (the reasons for this are
complex, but the basic idea is that round-trip tickets are often deeply
discounted while one-way tickets almost never are).

Bangkok is one of the world's major airline hubs, and the high level of
competition keeps prices way down. It has reasonably-priced flights to
the major European cities, and dirt-cheap flights to other parts of
Asia, so if you need to buy a ticket there it's no problem.

See if you can get a round-trip ticket with an adjustable return date,
which will almost certainly be cheaper than buying separate one-way
tickets. If you can't, then just buy the cheapest round-trip you can
find, don't use the return part, then buy a cheap return ticket in
Bangkok when you want to come back.