The Logo Joke is of Belt using his "Dun-Dun-Duunnn" catchphrase. He generally used that towards describing The End. But on the other side of every end, is a beginning.
Hope, Phil and Dawn = hopeful dawn.
Eep gets the Thunder Sister title of Fire Heart, and the Spiny Mandrilla is brought down with a heart-shaped skull by Eep and her lover Guy using fire. Heart Is an Awesome Power indeed.
Dawn gets the Thunder Sister title of Sister Sunset. It's actually a more suitable name than Dawn thinks: both dawn and sunset are polar opposites of twilight periods (when the sun is halfway across the sky) - dawn for the sun rising from night to day, and sunset for the sun dropping from day to night.
Thunk's inclusion in the Thunder Sisters seems incongruous, given that he's male. But he's also the only character in the film who has sisters, plural: Eep and Sandy.
The animals Grug saved in the first film are nowhere to be seen, with the exception of Chunky and Douglas. They may have just gone off somewhere on their own, but it's likely that as the last of their kind they died off.
Or even worse, eaten. By predators or the Croods, either is unfortunate.
Chunky and Douglas were the only animals to join the Croods of their own volition. The others only hitched a lift with Grug's "idea" to get away from the disaster; it's possible they were content to go their own way as soon as the travelers reached a habitat suitable for them to live.
