I don't know if I found training mode's easter egg, but I'm going to list a few things:
1) Switching characters while knocked with a gold kds, says you "commited suicide" because you switched when downed
2) Using Wattson, place pylon and stand next to it, and just switch characters. You may need items, gear, etc. but the effect that shows her ability charges carries to every other legend, except Crypto for some reason
3) Crypto is the only legend who's tactical is closer to the health/shields slot... I haven't checked if it's like this in the main game yet.
However, these things are the only ones that can really be done without that many requirements, first one requires gold knockdown, knocking yourself, and then switching. Second one seems to be a pylon charging glitch by placing a pylon, charging, and then switching. Although, it only seems to really affect Wattson. If you switch as another legend, it doesn't seem to transfer the charging effect next to the tactical button.
I don't know... I'll keep looking!
Edited: That game randomly restarted itself and placed me back at the spawn point after noticing the pylon created a double ring around it, and now the pylon suddenly reaches further away.
Edited 2: Dropping armor, switching legends, then quickly picking said armor back up, seems to turn down the audio for a few seconds. (Seems to happen when just switching characters next to a pylon)
Edited 3: Manage to glitch audio and it's making a weird static gargling sound... I don't know if something is hidden in it or what.
Edited 4: Switching legends when being hurt, refills shield and health, assuming you weren't downed.
Edited 5: Quickly switching characters delays their "I returned" line and a connection line error occurs for a moment.
Edited 6: - Switching from Mirage after using his ult, seems to place the next legend into the same vanished mode until it expires.
- Octane's speed affects other legends on switch
- Somehow manage to get lobby music (the slider) to be the music, unsure if it was always like this
Edited 7: Using Octane and then stimming, then switching to Crypto and using his drone causes a weird color effect when the stim wears off, and it's only for a short time. Doing it with Octane and then Wraith causes camera shake when the stim expires during her phase.