It always feels too simple and anticlimatic to just go up or down with nothing to tell we teleported except for the sound.
Adding a small poof of purple enderman-like particles to te position the player teleported or around the player should be agreat solution for this, I guess.
idea 1: have the particles come from the player, spreding in all directions. noot too many to not polute vision, and last about 1-2 seconds.
Idea 2: have the particles spawn around the player and stay in the air for a few seconds. Not too many to avoid visual polution.
Idea 3: have a few particles spray from the teleportation block and be somewhat affected by gravity.

Optional: Have a small trail of particles stay in the air behind a teleported player, like a "hey, i just got out of a teleport" effect.

I hope I'm not asking too much here, but I'll be glad if you could do this. Thanks!

I fully support this suggestion as well.