As for items being removed I don't really care that much since you can always choose to have them not appear. As for online they could have a setup similar to Mario Kart 7s where you can make groups that only allow certain items, or just have different modes such as "Normal" (Which would have all items on), "No Items," "Barrels and Crates Only", and things like that, and then there would also be Team Battle and Special Modes of course.
One thing that was mentioned in the video Krazy4Krash posted that I am really behind is the inclusion of alternate costumes along with alternate color schemes. Every character should have 1 alternate costume, and certain characters could have 2. Mario could be Dr. Mario, Donkey Kong could be Donkey Kong Junior, Link could have the Zora armor and Toon Link, Samus could have the Fusion suit, Power Suit and/or Light Suit (All costumes could turn into Zero Suit Samus), Pokemon Trainer could have the male and female variants, Wario already had an alternate costume, Olimar could be Louie, etc. Tripping should also be removed, obviously.
Playable Characters I would like to see added
Bowser Jr./Shadow Mario
Young Link (Majora's Mask)
Isaac (Golden Sun)
King K Rool
Ghirahim
Dark Samus
Shadow (But that's it for 3rd party characters)
Baby Mario and Baby Luigi (As a pair like the Ice Climbers)
Krystal (Star Fox)
Mewtwo (as alternate costume for Lucario or vice versa)
Waluigi
Roy (Fire Emblem)
SPOILER ALERT FOR KID ICARUS UPRISING!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Medusa or Hades (Kid Icarus)
Samurai Goro
Porky Minch