Aside from random rock jellies and that one slush puppy there are no monsters in LD.
I suggest creating a map based on Aurora isles in which there are two Gold Trojans (i.e. invincible) these Trojans would only attack one team. One would go after team 1, the other team 2.
Provide an extremely wide spawn exit and healing pads around the field. If a person is on a healing pad they can be targeted by both of the Trojans and they become their priority. If more than 2 teammates are on a cap they too can be targeted.
Unlike other monsters gold Trojans would quickly break up spawncamping trips and force players to worry about something they can't kill.
________________________________________________________________________________________________________
Why not create a Candlestick keep LD map where a lone Grimalkin wanders around.
Put candles and embers here and there and put healing pads in the dark.
_________________________________________________________________________________________________________
Other monsters such as Deadnaughts, gorgos, wolver packs, and lichen colonies can roam the map.
Put mini spawners, rocket puppies, and various polyps on the edges.
You could even make a room filled with all the (gold) wisp types, or a random battlepod.
I once thought of a swarm map, where there would be swarm turrets that defend captured control points from other teams. (If the control point was red, they would fire at blue. If the control point was blue, they would fire at red. If the control point was neutrual, they would fire at both) There would however only be a few bullets so that they can not kill on their own