Some say the writers were making a consideration of an absolute sequel to make a good series because of high rating they got from Season 1 and the disappointing outcome of Season 2, leaving a lot of cliffhangers.
1. The writers might add an alternative solution like Inaho and the rest of the team were fighting for the other 37 knights hasn't accepted yet the truce between Earth and Vers. It is already implied that some of them hated the Terrans to the point occupying some part of the Earth even though the Princess turn to become their new Empress, ordering them to give it back. There is a possibility of kidnapping again in this series that would break the peace once again. You know people of Terrans won't also forgive these people.
2. It would take two years before Slaine Troyard to be freed from his prison. It is possible for the writers to write about the mighty escape of Slaine from the hands of Inaho when the remaining six of his men such as Harklight and Count whatever his name will save him. his mission is to save the Princess bitc* cough* I mean his friend from the culprits.
3. Because Slaine was supposedly declared dead, Inaho will be the one who will stop him though, Slaine only wants to save the peace. Batman (Slaine) vs Superman (Inaho) something like that. The writers might possibly let Slaine won for the first time against Inaho from this series, making him too OP since he has learned a lot from his mistake. Frustrated and finally lose from a battle, Inaho will now desperately going after him. Likewise, Slaine only wants to protect the peace not to lead a revolution in which his other loyal men (commoners) will do their best to protect their lord.
4. Slaine might possibly be renowned as a Roque Count who will kill all the Counts and possibly corrupted Terrans that doesn't want peace.
5. Slaine's demise and cruel works will be given pardon after he saved the earth and Vers. You know presidential pardon, lol.
6. Some anime watchers and fans wants him to convert from Villain into Anti-Hero protagonist because of his true nature and capabilities. Also, they want him to become the next Emperor and married the Princess Nunnally, I'm sorry, I mean Princess Lemrina.
7. Slaines deserves a happy ending since he was considered number 1 favorite character in this series. No idea how but they don't like Inaho so much.