It might be more conclusive. While the previous light novels built the world and characters, the final one will wrap things up. It could have a different tone, perhaps more solemn as it nears the end of the story.
We can expect a thrilling conclusion to the story. It might tie up loose ends, like the relationships between the main characters. Maybe there will be final battles against powerful foes, and we'll see how Kazuma and his party have grown throughout their adventures.
Well, Beowulf's final fight was against a dragon. You see, this dragon was guarding a huge treasure. A slave had stolen a cup from that treasure, which made the dragon furious. So it started attacking the Geats. Beowulf, being the hero he was, couldn't just stand by. He gathered his men and went to face the dragon. But his men were mostly cowards. Only Wiglaf stayed by his side. Beowulf was strong, but age had taken its toll. He fought the dragon valiantly. They exchanged blows. Eventually, Beowulf was severely injured, yet he and Wiglaf together managed to kill the dragon. And then Beowulf passed away, a great hero to the end.
Battle Through the Heavens was a fantasy novel. The ending of the second part was not clearly stated. However, generally speaking, the ending of a fantasy novel is more fantastical and unexpected, so readers are advised to read this novel for more information about its ending.
One idea could be a crossover with Street Fighter. Imagine the characters from Final Fight teaming up with Ryu and Ken to fight a new, powerful villain that threatens both their worlds. The different fighting styles would create really cool battle scenes.