There are many design-related issues like routing, error control, flow control, reliability, scalability, security, and many more.
The main 2 issues are:
Addressing: According to https://www.internetworldstats.com/ there were already 4.5B internet users by June 30, 2019. At any given time, there are millions of data being transferred between a large number of users. If an addressing system is not properly implemented the data being transferred might land in the wrong location. A good addressing system should exist in the network protocol so that the senders and receiver are identified, and the data is transferred to the right person.
Security: Security is a major issue. If a good security system is not implemented the data might get lost, corrupted, or altered by the intruders. There should be a proper mechanism to protect the data being transferred.
How these can affect the network?
If the proper addressing mechanism is not used the data being sent over the internet might get lost or land in the wrong hand, which will cause a problem for both receiver and sender. The trust in the internet will decrease and the users will decrease and it might directly affect the economy. The data congestion might occur, if by any mistake all the address gets changed to the one address, there will be an overload which will report the failure of the network system. Without a proper addressing mechanism, we cannot consider the network reliable.
Security if not considered while designing the network protocol might arise many problems. If the proper security mechanism is not designed it won’t be capable to defend against hackers/intruders. Without security, the sent data might get altered.