Humans
For me, as a halo fan, I love the books, since I have read all of them, 343 should bring the remaining spartan 2's (linda, kelly, fred) and spartan 3's (naiomi, lucy, tom, ash)back into the halo lore of halo 5. Since Dr. Hasley is alive and escaped the forruner planet, that means the remaining spartans other than cheif are still alive. It is also said that that the spartan 3's and 2's were given the option to be in the spartan 4 program. As I said 343 should really bring these characters into the game.
Covenant
as for the covenant, the number one character that should returm is the Arbiter(Thel ' Vadam) the story would be much more expanded if they were to bring back the arbiter and his part against the civil war. As for the antagonist against the arbiter, 343 should make Jul ' Mdama(character from spartan ops) a more interesting character and explore his side as to why he hates the humans after their defeat.
Other
-Dr.Hasley
-Lord Hood(still alive and admiral of fleet)
-laskey
-sarah palmer
-spartan 4 fire teams(majestic, etc)
-didact
Please spread the word, and thank you