My suggestions (with explanations):
Assassin: Awilix, any other assassin - Awilix is just really fun to play, but I think to play her effectively you have to establish a certain level of comfort with assassins. Playing Thor and Bastet will help you establish that, and once you have and you're looking for something different, Awilix is a great choice. She also helps you move into more basic attack oriented assassins like Kali and Ao Kuang, because one of her most important abilities is only available during a basic attack combo.
Guardian: Khepri, Sylvanus - Khepri's kind of a no-brainer (although he recently got a health nerf). He helps establish a support mindest, while still allowing you to be active and feel like you're contributing in other ways. He sets up kills and protects his team with his grab and his root, and he can save teammates with his ult. You'll be actively looking to support your teammates, which is typically your goal as a Guardian. Geb is the ultimate support Guardian, but when I was first starting out I found him a bit boring and cumbersome. After mastering Khepri, I went back to him and found him much more fun.
I recommended Sylvanus because he's a very different style of Guardian. With the list of Guardians you provided + Khepri, you have a pretty standard set of Guardians. Sylvanus gives you the opportunity to try something different if you're bored of just taunting people with Athena all day. He supports his team by healing, he plants seeds to help restore Mana, and he has a root and a grab. He takes a lot of getting used to, so I would only try him out when you're feeling adventurous or need a change of pace, but he's a very interesting god.
Hunter: I barely play any Hunter other than Neith, so I don't have much of a suggestion here. Maybe Hou Yi. I think he and Rama might be the two best Hunters in the game right now, but I'm not really a student of the meta so I may be incorrect there.
Warrior: Bellona, Bellona, Bellona - No warrior list is complete without Bellona. She's just that good. As a second choice, I would go with my personal preference of Tyr, who is a bit difficult, but fun, to master. Amaterasu is also a decent choice.