Major international football organisations including FIFA and UEFA have either introduced new competitions such as the Nations League or are considering extending club competitions.
European soccer body UEFA has already decided on the competition format for the 2021-24 cycle, which will see the addition of a new third-tier club competition provisionally known as UEL2 as well as the second-tier Europa League.
There has also been some talk of potentially shifting the Champions League - Europe's prime club competition - to weekends to boost viewership.
"We are viewing the potential expansion of competition ideas very critically," Seifert, CEO of the German Football League which is in charge of the top two divisions, said in a speech to his organisation.
"That goes especially for any expansion of club competitions of international federations to what are today regular matchdays for domestic leagues, for example the weekend."
"Should this red line be crossed, then we would have to look at legal action... It would not be acceptable to shock the entire football structure with such actions."
Seifert is also the new chairman of the World Leagues Forum, which represents professional association football leagues.
"The weekend must belong to the national leagues. This is the heart of professional football," he said.
(Reporting by Karolos Grohmann; Editing by Hugh Lawson)
