This is really opinion, but I'd suggest `_` or `-`. Both can be used in interface names:
ip link set eth0 down
ip link set eth0 name eth0_sub1
ip link set eth0_sub1 up
ip link show eth0_sub1
2: eth0_sub1:
link/ether 08:00:27:0f:db:b3 brd ff:ff:ff:ff:ff:ff