Kael'thas has returned as one of the main characters recently, and his development is quite interesting, he was in fact a fairly popular character. If he redeems himself completely, and is able to leave the Shadowlands and return back to life, for example, do you think he should rejoin the blood elves and Quel'Thalas, as they are still his people and kingdom after all? (He even acknowledges blood elves and void elves as his kin).
This could make for very interesting and intriguing storylines. For example, Lor'themar Theron refusing to accept his return, and both he and Rommath blaming Kael'thas for his betrayal and everything that happened in the past.
Perhaps this causes a rift between them and Liadrin and Halduron, who believe in redemption and second chances, and this causes a power struggle in blood elf society between the two factions, one which wants to continue the Sunstrider dynasty, and the other which seeks to have it ended forever, which could easily go a number of different ways. Kael'thas's views on the orcs, trolls and Forsaken would also be interesting, as might Thalyssra's reaction to him should they encounter each other, not to mention Calia, the sister of the man who destroyed most of his people.
This might open up new and different possibilities, with Kael'thas's interactions with the three Windrunner sisters - Alleria, Sylvanas and Vereesa, all of whom must have known him - and other characters such as Jaina Proudmoore (whom he might still have lingering feelings for), Khadgar, and the members of Dalaran who were his friends. Alleria and her void elves might be conflicted on Kael'thas's return, some even tempted to return to Quel'Thalas with their people, as might Vereesa and her high elves, and I'm not sure how Sylvanas and Kael'thas would regard each other either. A fight between Kael'thas and Jaina, for example, would be fascinating, his phoenixes against her water elementals, and to see how their powers compare with each other at this point. Or even Kael'thas with Alleria, his devastating fire spells against her Void abilities?
Would it not be fitting if the blood elves - focused on redemption and change - eventually re-embrace Kael'thas as their leader, or at least accept him back into their ranks, or even if he returned to Dalaran and found a way to reclaim and re-empower Felo'melorn? People always complain that the Horde leaders are very weak and feeble compared to the Alliance leaders. So perhaps an iconic and powerful character like Kael'thas with Felo'melorn could rebalance the power balance between the factions somewhat? Perhaps he could even take control of Tempest Keep once again, and return it to Quel'Thalas, floating over Silvermoon and the Sunwell Plateau - like a counterpart to Dalaran and the Vindicaar on the Horde side?
What you think should be next for Kael'thas? Would he make a good leader for the Horde side?