My idea of a sad ending: Mickey tries to help Terra beat MX, but the DS keeps him busy. Aqua is in a dilemma over whether she should help Mickey, or leave Ven without help. Ven convinces her to place the safety of the worlds above his well-being, and Aqua painfully goes to help Mickey. Mickey and Aqua manage to beat DS (possibly killing him).
Meanwhile, Terra is busy trying to defeat MX. MX is winning, and gets cocky, and Terra is able to catch him off-guard. MX is greatly weakened by Terra's strike, and either attempts to absorb KH into himself to become all-powerful, or tries to possess Terra (no, keep reading).
Terra tries to stop MX, and KH explodes. The unpredictable nature of hearts causes MX to be greatly weakened, and this combined with Terra's presence changes his appearance, making him look like Terra.
OR, in the scenario of the possession, since MX was so weakened by Terra, Terra manages to push away MX from his body. This combined with his very fragile state changes MX's appearance, making him look like Terra.
Or, DS, presumed dead, tries to go and help MX. Right when MX is pushed out of Terra, he's hurled straight into DS. This causes an accidental possession.
Either way, Aqua and Mickey arive way too late. They go back to see how Ven is doing, only to see that he died too. Terra, already very weak and wounded, is devastated by Ven's death, and dies shortly afterwards.
Both are overcome with grief, and pledge that Ven and Terra's sacrifices were not in vain, unknowing that the evil they died to stop still lurks elsewhere.
Terra's grief over Ven's loss causes his spirit to manifest itself as Lingering Sentiment. As for Mickey, he goes back to DC to rule his kindgom. Aqua...haven't given much thought to her.
