You currently can't: https://twitter.com/McLeodGaming/status ... 7127103490

I'm assuming you had to use Sandbag's Final Smash to actually complete it.

Stuff we're currently missing that we know about;
Saffron City 64
Steel Diver
Venus Lighthouse (Postboned for Future Update)
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We gotta find the princess

I appear to have unlocked Waiting Room after leisurely playing around in Special Smash (specifically the hilarious nostalgic SSF1 mode), so I think that one's time or match count sensitive as well? It's strange because another user appears to have unlocked that one before World Tournament whereas it was the other way around for me.

It could be because I've played a large number of mostly short matches, so whereas World Tournament could be match count sensitive, Waiting Room could be time sensitive (and since it's about waiting, that wouldn't be a big surprise). I can't be positive of course.

Tl;dr just group smash a lot and you'll get both Waiting Room and World Tournament.

EDIT: Unlocked Saffron City 64 by clearing Classic mode with Jigglypuff (didn't do it with Pikachu, dunno if it works for him too).

Surprisingly, after clearing event 36, you unlock the Master Ball item

All of a sudden, I just unlocked alternate music tracks

actually u unlocked desk not sandbag, thats a bug

According to 194, play 15 matches for Alt. Tracks

Kill yourself many times (i did 25 times) on Lake of Rage to unlock Steel Diver.

Given all that we know how to unlock, there's still room for one or two more stages on the stage select menu, that may be wishful thinking though.
I doubt there's any new characters or anything, so stages and potentially items are what we have left?

Considering that there was a stage for completing hard classic mode there is probably a stage for completing insane classic mode.

Was it jumping into the water, or just walking of the edge?

That's definitely not true. I've hit over 50 at this point and still nothing.

I got Steel Diver from the water thing at Lake of Rage, but I think it's either related to splashes or amount of time spent underwater. Would probably also work on Gangplank Galleon.

Also... I think Waiting Room is simply by playing a match with Sandbag. At least, that's how I got it.

I've cleared it on Insane, but I had to use continues because the Metal Bros. just mess me up. Don't know if clearing it without continues would cause anything different to happen.

Both

You must not have to play as Sandbag then, because I certainly did not when I unlocked this stage.
I forget if he was even in the match I unlocked Waiting Room in. I doubt it.
Also:

I cleared Classic on Intense without using any continues with Black Mage. Got Metal Cavern and nothing different.

I also managed to clear it as Luffy on Intense after unlocking Metal Cavern and got nothing :/