I believe Melee HD is the only thing that could kill the melee community. The only thing Nintendo could do to make a Melee HD and have everyone switch is...

1. Have all physics, hitboxes, etc work exactly the same. No increases damange, no nerfing fox, no buffing low tiers or even fixing G&W's shield. (Trust me, Melee needs changes, but if you let nintendo change it, it will be more than 100% agreed upon changes, it will divide the community, and melee might die.
2. Having beautiful updated graphics and sound. While still keeping the melee ascetic. (Most melee people on know like the style of characters in melee more, than brawl or Sm4sh, it should be a detail update of the characters, not a style change.)

These above are the main two, it is most important melee HD does the above.

1. Having costumes (only reasonable, non distracting ones, like this), and maybe some stage skins, but that is much more dangerous, and so much be done carefully. Costumes should be in online play, and there should be an option to see everyone as normal, non costumed characters. Costumes should have no gameplay affect and are purely ascetic.
2. Online play, we already have netplay, and it's really good, so it has to be as good as thank (It frankly can just be that), we know melee has inherent lag, and that is actually a good think, it allows online play while having the exact same lag as console. It should have a ladder system too.
3. It would be great if Melee HD could include 20xx training features, ucf, recordings, etc. But there must be a toggle and allow 100% vanilla melee to be played. To help us, nintendo could even make it more apparent which version we are playing, and even show something small in game like near the character icons or percents. It should have 4 obvious modes, Normal, Tournament, UCF, 20XX. It should be clear at all points what mode you are in. Tournament mode should involve nintendo talking with TO's, and make it so that things like damage percents are impossible to change in tournament mode.

Lastly, I think Melee HD could make a ton of money by simply letting people buy custom skins and use them online (or even in tournament). People would pay to get a c9 skin, tournaments like genesis could offer special skins for coming or getting out of pools, that you can only get by going to the tournament, then you could use online or at later tournaments. Trust me, people, including me, would eat that up.