As long as Nintendo prepares the character's mesh, animation, texture and character properties data to be capable of being loaded from an external source (such as the Wii's internal memory or from a Memory Stick), it is more way more than *technically* possible. Things like all new engine mechanics, such as let's say "Ditto" being able to dynamically morph his mesh to become another character's mesh would be tricker when you're talking external implementation though only because you're talking about a system not built into the Game Disc. Again though, it is *technically* possible.
The unlikelihood is not from the technical standpoint, but from the company, Nintendo, and its own standpoint on using modern capabilities to benefit game play. They seem to be a generation behind in that area, so unfortunately I'll have to be a pessimist and say that we'll see a hint of this sort of thing in SSB4 for the Wii2, and actually see it used in SSB5 late in the Wii3's development.
Were they to use external implementation to add in new characters, certain drawbacks would include the fact that every user who plays Brawl on-line would have to have a storage capacity either on his Wii or in a Memory stick to include the maximum space required for all of the new additions. Each new addition could easily range from 10-100 megabytes a each depending on the complexity of the character. Other drawbacks would be people hacking their copy of Brawl to try and force-feed their own character data into Brawl, which would require checks and balances on Nintendo's side to make sure this wouldn't filter into on-line game sessions.
The maximum amount of characters I could foresee being added through this method would be 2-8 characters over a period of 2-3 years. This relatively relaxed release schedule would allow for a small team (perhaps Junior members?) within Nintendo to make the balances and resources(art,coding,etc) required. Sakurai would ideally be overseeing the process and making the final decisions on the new characters, but with a casual deadline and a short contract that would only include two weeks of his time to oversee each addition as necessary.
I'd like to say in closing though that Downloadable Characters/Content was the first thing on my mind as the character roster was being confirmed, as it would allow for a second chance for any of those characters that just couldn't make it into the game in time. (Geno)
The only problem there is that if you want to play online, everyone playing against you will need the same characters you have downloaded. Forcing you to constantly download every character they produce, just to be able to play online.
Plus, even if it would happen I expect Nintendo to charge money for it :P
its pretty much near impossible anyway. the wii's HDD is only 512mb. the game is 128 blocks big as well and a DLC character would be about 90-110mnb. most of the spaces are taken up by other games and channels as well, so unless they release an extrernal HDD then its not a real solution. WE just have to see what the futere holds for the Wii's space development
You can only play online after uploading the 'latest' characterversion (which goes automatic)... also...they could 'force' us in buying SD cards...I mean, look at the GC, they could force us in buying memory cards back then,so why not now?
I personally believe that the chances of getting new official characters are very slim...but I have high hopes on the mod'ing community...seeing as the wii is easily accessible...they could make mods for new characters (who can be turned on and off when you want to play online with people who dont have that mod..)