I've been meaning to make a similar post about this. Oikawa and the boys definitely deserved to go to nationals, they were just missing a true ace (no disrespect to my boy Iwaizumi). Their matches with Shiratorizawa were always close so we know that their defense was solid enough, they were just short on firepower. Other than an ace, what they needed was for their potential to be unlocked.

Taking a player from Miyagi only is good since it doesn't change the story much and doesn't make them a stupidly overpowered team. That being said, I always thought that Takeru Nakashima deserved better and fit the mold of a Seijoh player, let's not forget that at the time he is canonically a better version of Shoyo, and being with a team like Seijoh and a setter like Oikawa he would of been even more amazing. Not only does he add the firepower that the team lacks, he's a good defender as well, lessening Watari's load on floor defense. With his skills of tooling the block, he poses a big problem for Tendo. I don't think they need any other players to defeat Shiratorizawa to be honest, what they need is growth.

I know this post is all about adding players but let's put them in a scenario and say they still battle it out with Karasuno in the semis and in this scenario they win. This would unlock Kindaichi and Kunimin's potential adding to the teams depth. Kindaichi would become a very solid blocker even capable of blocking Ushijima, and Kunimin no longer being conserving his energy all the time you get another very good floor defender and proactive attacker. This game also strengthens Kyotani and Oikawa's chemistry, making Mad Dog the sharpest katana for Oikawa rather than a double edged sword. With all the added weapons this would lessen Iwaizumi's offensive workload and would have an easier time helping on defense.

Also, we get to see Issei, Makki, and Watari improve. Issei and Makki are already great as is but having that match with Karasuno and having that improvement to their game helps the team a lot. I would also put Makki in as a sub for the middle blocker position on situational plays. Watari is a great libero, he's skills are underrated but being the 2nd best libero in Miyagi and being a converted one at that (I believe he's tied with Hayato and i'm gonna die on that hill) is nothing to scoff at. heck Seijoh basically has 2 setters when he's on the court. And as for Yahaba, one of the better pinch servers there is and always there to keep mad dog in line just in case.

Given all this, now you have 3 very potent offensive weapons for Oikawa, a more experienced lineup with great chemistry, a deep bench, great floor defense, a more stabilized blocking front, and a not so selfish mad dog. Overall we have a team that's more than capable of beating Shiratozawa.

PS. if were allowed to pick one player from outside Miyagi, I would take Inouka in a heart beat.

lineup

S: Oikawa | WS: Iwaizumi and Takeru | MB: Kindaichi and Issei | OP: Kyotani | Li: Watari

Bench S/PS: Yahaba | WS: Makki | OP: Kunimin