The spell banishment allows you to banish a creature to a harmless demiplane, where they remain incapacitated for the spell's duration:
You attempt to send one creature that you can see within range to another plane of existence. The target must succeed on a Charisma saving throw or be banished.
If the target is native to the plane of existence you're on, you banish the target to a harmless demiplane. While there, the target is incapacitated. The target remains there until the spell ends, at which point the target reappears in the space it left or in the nearest unoccupied space if that space is occupied.
The idea here is that I want to be able to send myself or an ally after the banished creature to inflict further harm upon them while they are incapacitated.
I considered a couple obvious planar travel options, but came up empty handed. The gate spell seemed a bit too ambiguous to be a reliable method:
You conjure a portal linking an unoccupied space you can see within range to a precise location on a different plane of existence.
I think the challenge here is that you don't really know where the creature is, just that it is on some demiplane, so you can't really pinpoint it as a "precise location". Plane shift requires a fork tuned to the destination plane, and a teleportation circle if you want a precise destination within that plane.
But perhaps there is a more subtle and clever wombo-combo of spells and magic items I am missing. Is there any way to follow a banished creature to the demiplane destination of banishment?