When it comes to DKCR2, I think they should add Dixie, but add Funky as well, using his bandana/muscle shirt/shorts version as seen in Donkey Konga 3 (Japan-only) and Mario Kart Wii (hidden character). It's not "too '90s," and Funky could be a functional Donkey clone, meaning that in four-player, players could pair up for tough spots, or in two-player, each player could control two characters (and in three-player, one player can control up to two charactes, while the other two players each control one). Again, DK and Funky would function the same, and either could be mixed and matched with Diddy and Dixie for unique "power-ups" (i.e. rocket barrels with Diddy, ponytail twirl with Dixie). This would work great, but seeing how tough two-player co-op can be in this game, the levels would have to be especially brilliant in their design to comfortably accomodate four...
When it comes to animal buddies, I think a sequel should focus on Enguarde the swordfish, who is probably the most famous animal buddy after Rambi. Of course, this would mean bringing back underwater levels. If they can find a way to make underwater levels fast and fun, I'm all for it, as it'd be a huge addition and add a whole new dimension of gameplay and abundant opportunity for creative design.
Squitter would also be a delightful addition, spinning midair web platforms and firing web bullets. Perhaps he could even have a new ability in the form of scaling walls and ceilings for upside-down platforming? Any of these powers would handily distinguish him from Rambi.
The other key ingredients for a DKCR2? A snow world -- we need to have a snow world in the sequel -- and KREMLINGS! Bring back K. Rool and his cohorts, and maybe even have a world or two where they're pirate-themed, maybe with a mid-game encounter with K. Rool in his Kaptain form (like encountering Kaos halfway through DKC3). Lay the dark, moody atmosphere on thick -- perhaps a trip to the Kremling homeland is in order? -- and maybe have Yamamoto and Wise collaborate on a track, this time with some authentic orchestral/choir backing.
And online co-op multiplayer with WiiSpeak/Headbanger headset support. Yesss...