Thank you guys for your replies. 
Okay, there should be even more options for sure, but I did not think about it before.
A bit of a summary,
1) You can burn the unsold tokens
2) Keep the unsold tokens
2.1) Airdrop them
2.2) Give them to the founders
2.3) Use it in a reserve fund to incentivize the usage of the token, for example, by promoting the usage of tokens (eg. reduce fees during a certain period)
2.4) Use it for bounty programs?
Of course, 2.3 & 2.4 must be specified in the White Paper beforehand.
What do you guys think about the other options (2.3 and 2.4)?
Okay, there should be even more options for sure, but I did not think about it before.
A bit of a summary,
1) You can burn the unsold tokens
2) Keep the unsold tokens
2.1) Airdrop them
2.2) Give them to the founders
2.3) Use it in a reserve fund to incentivize the usage of the token, for example, by promoting the usage of tokens (eg. reduce fees during a certain period)
2.4) Use it for bounty programs?
Of course, 2.3 & 2.4 must be specified in the White Paper beforehand.
What do you guys think about the other options (2.3 and 2.4)?