Nintendo doesn't want to make competitive games, and certainly not competitive fighters. They will probably leave Melee and 64 to languish as historical oddities in the big book of dead fighting games.
It's too bad we can't jump ship to anything else and sever all emotional ties, because there's nothing remotely similar to Smash Bros out there. All we can do is continue to play a game that will never be further developed or supported.
Best possible scenarios:
-Nintendo decides to create a side-branch of games in the Smash-bros series based off Melee and 64 gameplay, for the competitive crowd, separate from the main series.
-Or some other game developer decides to make a new series of fighters using the Stages, Percents, stocks and ringouts system instead of depleted health meters.
I suspect eventually the second one will happen, but it could be 10 years. In the past, for other dead games, the fans have stepped in and filled developers shoes. Total Annihilation is a good example. Their developer (Cavedog) went bankrupt, so from then on they had no online service and no new game support. They made their own hack-based game: Spring, which allowed them to hang on until another studio released Supreme Commander, a new series based on similar gameplay.