Any vehicle (for instance, a minecart) that enters a portal should transport itself and its passengers to the applicable destination, possibly subject to a config option.

This is a legacy feature that we suspect broke in 2016.
A restoration of this feature has been highly suggested by the community.
This will close upstream issue PseudoKnight#9.

For entities, this could possibly include velocity/bungee support, resolving DrakiaXYZ#14
This would involve duplicating the entity, spawning it in the other server, and deleting it in the origin server.
It would definitely need a config option though.