Does anyone know how to make a map go straight to another map upon completion (in a mod)

easiest way: use a trigger_once, triggering a point_clientcommand with the command changelevel <name of the next level>

If this is a portal 2 mod, you can use a point_changlevel to change the map.

Alternatively, you can use a trigger_changelevel, a trigger_transition, and an info_landmark to keep the player's motion relative between map switches, if you aren't using an elevator. There should be some good tutorials on YouTube and the valve dev wiki

Thanks!