A powerful, ancient sorcerer could be a great main antagonist. This sorcerer could have been locked away for centuries but has now broken free and wants to take over the universe from the Master of the Universe. He could have dark magic that the Master has never faced before.
Perhaps an alien race that is extremely advanced in technology and believes that the Master of the Universe is a threat to their expansion plans. They would use their superior technology to try and defeat the Master. Their ships could be equipped with weapons that can disrupt the Master's powers, and they would be highly organized and strategic in their attacks.
A fallen angel - like being could be the main antagonist. This being was once good but has been corrupted by power and envy. He sees the Master of the Universe as an obstacle to his own rule over the cosmos. He has his own set of powers, like the ability to manipulate the minds of others, which he uses to turn the Master's allies against him and create chaos in the universe.
It could be a corrupted Gem that has the ability to disrupt Steven's new ways of sensing the world. This Gem might be able to create illusions that confuse Steven's other senses, making it difficult for him to rely on his friends or his new powers.
Another possibility is a new character that is unique to the other universe. This character could be jealous of Harry's arrival and the power he might bring from his own universe. So, this character tries to stop Harry from exploring and potentially changing the new universe.
It could also be an old and powerful magical creature that has been awakened by some dark magic. This creature sees Harry as a threat to its existence because of his mastery of magic and decides to target him. For instance, a dragon that has been enchanted with dark magic to be more intelligent and malicious.
It could be a rogue ninja who has also mastered some form of insect - based jutsu and wants to use it to take over the Hidden Leaf Village. He sees Naruto as a threat to his plans since Naruto is becoming the master of insects.
Perhaps an ancient spirit that has been awakened by the imbalance in the life - death cycle due to Harry becoming the Master of Death. This spirit sees Harry as the cause of the chaos and tries to destroy him to restore the balance in a way that is harmful to the wizarding world.
A powerful and evil Pokémon trainer could be the main antagonist. This trainer might have a team of extremely strong and rare Pokémon that he has trained in unethical ways. He could be after some sort of power or rare item that would give him even more control over Pokémon, and Ash stands in his way of getting it.
The main antagonist could be Master. Since the title mentions Master, it's likely that this character has a lot of power and influence in the merchant world. They might be using their position to cause harm or manipulate things in a way that leads to death, which would make them a natural antagonist for Harry and Tony to oppose.
A powerful dark wizard who was once a member of Fairy Tail but was banished for using forbidden magic could be a great antagonist. They hold a grudge against the guild and especially the guild master for their expulsion and now seek revenge by trying to destroy the guild from within.
Perhaps a group of anti - technology extremists could be the main antagonists in an Ultron fanfic. They see Ultron as the symbol of all that is wrong with technology and are determined to take him down, no matter the cost. They might use their own advanced, but more rudimentary, technology to fight against Ultron. This would create an interesting contrast between Ultron's high - tech abilities and their more basic, yet effective, methods.
It could be an evil version of the Sage of Six Paths. He might want to take the Juubi's power from Naruto for his own dark purposes.