Well, what kinds of monsters would be most interesting for your group to catch? I figure high level devils, demons, celestial, and dragons can reasonably find ways to plane shift. Additionally any creature smart enough to have class levels may posses the capacity to plane shift, as well any creature that might use a magical device to plane shift. Aboleths, mindflayers, djinn, salads, and neogi may all be reasonably able and have reason to plane shift. I imagine certain gith also possess the ability. Inevitables can also do so when motivated.
Sorry I don't have concrete specifics from the books. But are they using this net to try and catch something specific? Knowing what you're players are trying to accomplish might help lead to some interesting considerations.
EDIT: Oh! What if this net also interfers with summons? So a spellcaster tries to summon a creature from another plane, but it instead gets sucked up by the net? That could lead to some interesting options, as well as a pretty pissed off wizard and confused creature.