I've heard some rumors about a different intro video, etc. after you unlock everything, but that just isn't the case:
Brawl 100% Completion: Intense Boss Battles
100% SSE (nothing)
700 stickers (trophy)
544 trophies (notice)
258 songs (trophy)
128 challenges (nothing)
41 events (nothing)
21 Co-Op events (notice)
12 masterpieces (trophy)
5 unused golden hammers (nothing)
3 stage builder parts (nothing)
See also:
Brawl Challenges [Spoilers]
Brawl: All 700 Stickers
Brawl: All 544 Trophies
And a great resource for a list of all the known Notices:
http://brawlnotices.tk
Brawl 100% Completion: Intense Boss Battles
100% SSE (nothing)
700 stickers (trophy)
544 trophies (notice)
258 songs (trophy)
128 challenges (nothing)
41 events (nothing)
21 Co-Op events (notice)
12 masterpieces (trophy)
5 unused golden hammers (nothing)
3 stage builder parts (nothing)
See also:
Brawl Challenges [Spoilers]
Brawl: All 700 Stickers
Brawl: All 544 Trophies
And a great resource for a list of all the known Notices:
http://brawlnotices.tk