I can assure you it will never happen.
There are several reasons for this:
As it stands, PM is already in a very tight, awkward spot with not official support or recognition from Nintendo.
Luckily, one needs, in most cases anyway, SSBB to play the mod on their Wii so sales have seen a marginal spike around PM patches and releases. There are also cases where people even buy a Wii or new GC controllers JUST to play the mod. If the mod had the game by itself it would be a straight pirate, but it doesn't so it does not concern Nintendo too highly.
However, as soon as PM incorporates any new technology from Sm4sh, there will be drama as Nintendo's product will then be directly in competition with the mod as people might shift away from Sm4sh to enjoy the content elsewhere - a substitute.
The only reason, again, why the creators of PM haven't received a Cease and Desist letter is because no real monetary harm is being done.
When it comes to copyrighted material and intellectual property, there is a lot to be said so it is all a legal head ache in the end. There are many, thousands of licensing standards the mod doesn't follow because it isn't trying to do that, it's just trying to recreate Melee. But, for example, you can't just take Sonic and make him any color you want because there are huge trademark infringements that run along with that. It's just a mess, really, to get into.
If PM stays where it is and improves itself through balance patches and feedback from users, then it will surely prosper.
The day Bowser Jr. or Robin or anybody new enters the mod will be the day Nintendo will strike.