A few off the top of my head, and countermeasures.
1Torrent clients. if you have a security appliance to recognize and block torrent protocol traffic, use it. Torrent clients often use random ports. Running torrent clients automatically upload, which is detected by the ISP.
2 Illegal use. Someone could for example connect from outside and use the network for fraudulent activity. Use a password, temporary if possible. have good traffic monitoring in place and keep an eye on the DHCP logs.
3 Hackers. The router/modem can have vulnerabilities or weak passwords. If the network is not separated properly, guests could have access to sensitive devices. Update infrastructure regularly. Use a separate line / port on modem or strong VLAN and run something like Snort.
4 Denial of service. IPv6 RA flood will kill a lot of devices. If the VLANning is done by one device, it could be overloaded by another network. Again, monitoring or IDS will help this.