Crazy-ass theory, Minato Namikaze ( and by extension Naruto ) is part senju.
Following the confirmation of my '' Minato is a sage '' theory I'm raising the bar and shooting higher.
I truly believe Minato has Senju heritage.
Now, before you get your knickers in a twist, let me explain.
1. First of all, Naruto's been noted to have the Will of Fire, the ability to contend with Sasuke's Curse of Hatred. He has also been likened to Hashirama.
Now, it seems odd to have a half-blood Uzumaki fight a pureblood Uchiha and compare them to Hashirama and Madara.
The one that made that comparison was Obito Uchiha ( under the guise of Tobi/Madara ). If there's anyone in this world that would know where Minato came from, I guess it's his favorite ( Minato told Obito he will become a hokage one day ) pupil.
2. Minato's fantstic chakra reserves. For someone that isn't tied to any known clan that is known for their chakra prowess ( Senju, Uzumaki, Uchiha ) Minato has very high chakra reserves- this was seen with him summoning Bunta ( a kid Naruto could not do so with his base chakra alone, he needed help from Kurama ), porting away Juubi's bijuudama, porting away the whole alliance and various other feats.
Furthermore, he's seemingly a perfect Sage.
Now, do I need to remind y'all Orochimaru and Kabuto could not learn Sage Mode without getting radical DNA enhancements?
Look at Kabuto, the guy had to implement Karin's ( Uzumaki ) DNA, Orochimaru's and Juugo's to be able to use Sage Mode.
3. Minato's physical prowess.
We've see Minato manifest extremely high speed with shunshin alone ( arguably fastest base shunshin in the manga ), cut through Bee's tentacle with a kunai and manage to intercept the Kyuubi's claw after using RDS on himself and becoming a Jinchuuriki.
But that's not all, with a giant claw in his chest, the guy also sealed his, Kushina's and Kyuubi's chakra inside Naruto. He sealed half of the biggest 1-9 Bijuus after being stabbed through the heart by a giant claw and having his soul sundered by the Shiki Fuujin.
Do those durability feats remind you of anyone?
4. Minato and Naruto both mastering Sage Mode, whereas Jiraiya did not.
Now, this may be a farshot, but Hashirama is the first known shinobi to master Sage Mode.
Jiraiya, even after 30 years of training with the toads, could not master Sage Mode, whereas Naruto and Minato did. Coincidence? Skill? Perhaps.
But I don't believe in coincidence. How could Naruto and Minato do what Jiraiya could not in 30 years? Simple.
Being that Hashirama was the first Sage, I would think the Senju gene carries an increased affinity for senjutsu.
Considering Naruto and Hashirama ( and, by extension, Minato ) have the same talents- incredible chakra, durability, Sage Mode usage- I wouldn't be surprised to find out that they inherited Hashirama's affinity for senjutsu.
And yes, Kabuto attained Senjutsu without having Senju DNA, but he has Karin's, which, being Uzumaki, is watered-down Senju DNA.