How strong would Naruto be if The Masked Man never attacked the village? That means, Minato and Kushina would both be alive meaning that Naruto would not have the kyuubi.
There are obvious disadvantages to this, but this situation affords others that could outweigh what he loses. While we all know that losing the kyuubi is a drastic loss, its completely possible that Naruto would still have access to his chakra.
Reason – His whiskers. Naruto spent nine months feeding off of the kyuubi’s chakra while Kushina was pregnant with him. Kinkaku and Ginkaku spent only two weeks inside the kyuubi but we saw that Kinkaku was able to go into tailed beast chakra mode two, all the way up to six tails.
Naruto had access to the kyuubi’s chakra 18 times longer than the Gold and Silver brothers, so its safe to say that Naruto would still be able to use some form of the kyuubi’s power. Growing up, Naruto would have been able to learn ninjutsu a lot easier and would have been regarded highly.
Reason #1 – Chakra control. When Minato first sealed half of the kyuubi inside himself, he remarked about how heavy the chakra felt.
Early in the story, Naruto had intense trouble with his chakra control; the most obvious example is his difficulty with the clone jutsu. But, without the other half of the kyuubi’s heavy chakra, it is safe to assume that Naruto would have an easier time controlling his own and the chakra he inherited from the kyuubi during his gestation.
To back up this claim, we can again refer back to the Gold and Silver brothers and their fight with Darui. After Ginkaku got sealed, Kinkaku easily accessed the ninetails’ chakra and entered tailed beast chakra mode 2, albeit without the skeleton. Despite willingly submitting himself to the chakra, Kinkaku retained his consciousness and acted of his own accord. Based off of this precedent, it is possible that with time Naruto would be able to do the same with greater results.
Reason #2 – Training at home. First and foremost, his dad is the 4th Hokage. So from a young age his will be under the direct tutelage of best ninja in the village. Second, his mom is one of the last remnants of a legendary village, she is without a doubt, a basin of knowledge and sealing jutsu.
From this training, I think Naruto will have taken away a couple of key new talents, skills, and knowledge.
Speed – Minato was nicknamed the Yellow flash.
While it is bawdy to expect Naruto of having the same level of quickness, I think it is safe to assume that he would be naturally faster than he currently is based off of whatever in-house training he would give him. E.g weights, windsprints, etc.
Rasengan – With the help of Minato instruction and his increased chakra control, Naruto should be able to make a rasengan without the help of shadow clones. Whether or not he still comes up with the many variations of rasengan that he has become known for is up in the air, but I think that he not only maintains a plethora of rasengan’s, but refines them even further.
Sealing skills – We all know that Kushina is from the Whirlpool country, specifically the Uzumaki clan, noted for their sealing jutsu. But Kushina left the village at a young age, and it is entirely possible that she missed out on learning all of her clans secrets. But, the problem with this is, once again, Minato.
How did Minato learn the Death Reaper seal, or gain knowledge enough to create intricacies within Naruto eight-trigram seal? Answer? – Kushina. Kushina would have been able to teach Naruto everything she knows, which probably includes plenty of great seals and powerful barriers.
I think that I have rambled on enough but I think that I explained my argument well.
Suffice to say I believe Naruto would naturally be on a level higher than that of Neji and maybe Sasuke ( I guess in this world Sasuke could have continued learning from both his Dad and Itachi, taking him to even greater heights) had his parents survived. Minato was regarded as a genius from an early age so I think Naruto would also be a little more Shikamaru like as well.
Although our current Naruto definitely has a higher ceiling than, I guess we could call this make believe Naruto “Menma”, I think that Menma could be an absurdly strong ninja in his own right (evidenced by the RTN movie), based off of the huge jumpstart he would have had over Naruto.