There's lots coming on the horizon for Mario Kart 8, and Smash Bros U , both on the wii u.
First up, is Animal Crossing X Mario Kart 8,the second LDC pack for Mario Kart 8 [I highly suggest getting both packs to get the bonus Shy Guy & Yoshi color swaps). The pack will be released on April 23, and will feature Villager (male and female), Isabelle and Dry Bowser. There will be more karts 'n' parts, two cups (Crossing and Bell), each having 4 courses each.
And, on the same day there's an update for the game which will feature a 200cc mode which will be crazy fast (and allowing for more shortcuts...) and other goodies.
For Smash Bros, if you already have both the 3Ds and wii u versions, you most likely have Mewtwo by now. If not, he will coming April 28, and Lucas will be coming in June sometime.
Currently, there's an update for Smash which includes more mii outfits. Some you can also purchase for the 3DS version, as well. I gotta say, my mii fighter in vampire clothes with Majora's Mask on looks pretty sweet....
*I'm mostly familiar with Smash on wii u, not 3DS. There may be stuff happening for the 3DS version.