If the license of the original set allows it, it is OK to build upon it and improve it. There is no need to ask the original author (although it certainly would be polite) - some of these people would be hard to reach at this time.

No work of art stands alone, every artist gets their inspiration from somewhere. The better the artist the less obvious is the connection. While I agree that it is better to do larger leaps, it is perfectly OK to do smaller steps when one begins.