I doubt such plugin can be made (one that is easy to use and works with all types of games). But unreal has great AI system/tools. Behavioral trees with EQS (environmental query system) can be used to make formations. So it is kind of built in “plugin”.
With EQS all you need to do is simple blueprint function that gives scores to locations around target/leader pawn (or any actor). For eg pawn 0 in formation is leader, pawn 1 should be on left side of leader, pawn 2 on right side, etc. Then filter that function trough EQS system and you have your formation.
Back to plugin idea for a bit, i try to describe why such plugin would be extremely hard to create. For eg. you have formation working on plain/flat surface, now you want to add obstacles avoidance, and use some types of meshes as covers. So you need some hook to plugin to let it know that this barrel is bad cover (red that always explodes in games since duke nukem or so), and green barrel that is good cover. For such stuff you need some lists of obstacles, traps etc. But then you want to add spawning spot, if plugin has that implemented you are lucky, but if it does not your AI will not know how to react to spawn points. I think you cannot do just formation plugin, you need to do whole AI system, and every game is different.
I’m working on an RTS group formation movement plugin using my own pathfinding system. It is quite complex, and will be ready only next year, and cannot say that it would be cheap…
By the way on the marketplace there is a plugin or blueprint available doing some simple formation movement using navmesh. I do not know its quality and performance, could not get it even from the the developer…
In Unity check Apex Steer (improved version of Apex Path), that seems to be good for that price.