Nemona is the perfect female companion. She can be both a rival and a companion for Ash. We had Kiawe and now Iris, which are considered rivals for Ash, beside being companions. Seeing Nemona and Ash in a journey, both being obsessed with battles and achievements would be something fresh as we have not had a companion like that yet. On the other hand, Arven's backstory provides a lot of room for character development. Penny would also be something fresh, revealing herself to be the leader of the (somehow) villainous team, something we didn't have yet for companions. Iono would round the group so perfectly, by being the hype companion, famous and energetic influencer and a great battler.
While Rika from the E4 can be one of the rivals. She seemed like a cool character.
I wouldn't mind if Goh and Chloe will be the main companions as long as the development is great, however I would prefer everyone mentioned above, cuz they might provide a more interesting story and series.