Not only he's the main writer of nine seasons following each other (Goggle V through Fiveman), but, during that era, he introduced plenty of new elements that were gonna change the sentai franchise forever.
Hirohisa Soda has been a secondary writer for Super Sentai since Goranger.
However, when Shouzo Uehara left the sentai franchise to work on the Metal Hero franchise, Soda became the main writer.
Up to Sun Vulcan, Super sentai seasons were pretty episodic: the first part introduced the villains's plots, how the MOTW were attacking Japan and the consequences of their plot. Usually, there were guest characters that had an important role in the episodes, and were interacting wiht the heroes (in a lot of cases, at least one of those guest characters was a kid). The heroes were mostly fighting the MOTW, the mooks, and the lady fighters (like Salome in Battle Fever, or Amazon Killer and the Zero Girls in Sun Vulcan). . Each episode was pretty self contained, and there was no story arcs until the last few episodes of the season. Moreover, the villains were pretty one dimensional.
Let's review each Soda sentai series to see how many changes he has brought
Goggle Five was his first series; it was pretty following the trends shown previously; however, already, there were two commanders heavily involved in the action: Deathgiller and Mazurka, who were fighting the heroes in many episodes. Besides, the villain casr was more diverse, with an interesting hierarchy and different roles. Goggle V's tone was also much light hearted than the previous series, notably, with the kid main characters (Computer Boys and Girls)
Bioman was also the first series with two female heroines, and the first non Pink heroine; and what an heroine: she's the most important protagonist character after Red One, and she has a lot of focus, with plenty of action (and she's especially highlighted in the Magne Warrior arc (or I should say, both (but Jun had more screen time, so I'm gonna mostly talk about her). in Bioman, the female rangers weren't anymore the "girl of the team", but they had more diverse roles.
It can also be noticed that the main characters had very distinct personalities, with plenty of focus, and strong rivalries with the villains.
btw, Flashman was the first series with a second robot.
Maskman was the first series having each hero having his individual mecha
Liveman introduced even more new concepts. It was the first series with a blue female ranger, and the first series with animal-themed mechas, and the first having two robots being able to combine into a bigger robot.
But , more importantly, Liveman had an unique main story. For the first time, it can be noticed that the heroes and the villains were from a common background, as university students, and were former friends. As a result, the conflict is even more personal and painful, with a real sadness, because they were seeing people they knew becoming their enemies and were forced to fight against them. The theme of revenge was very prominent, since the heroes lost a lot of people they cared for (and for two of the heroes, siblings), because of their former friends's betrayal. However, the desire for revenge is mixed with a desire to save their former friends, and the consequences of choices that are made is an important theme. A lot of episodes deal with th heroes's desire to see their former friends realize their mistake, and stop being evil and look for redemption (it worked in one case, since one of their enemies becomes their friend again). More than ever, rivaliries between villains were highlighted. Another huge theme of the series is the theme of knowledge and intelligence and how science should never overshadow ethics and morality.The ending is definitely bittersweet, and there is a feeling of an huge waste of lives.
Liveman was also the frst series to introduce midway through the series regular additional heroes among the core team (Big One in JAKQ is a special case, since he's more of a commander replacement)
Turboranger even introduced more new concepts. It was the first series with a strong fantasy feeling, with fairies, demons, fantasy creatures. Moreover, it introduced the concept of high school heroes, and their school life was shown a lot. Once again, the heroes had enemies being with them in high school (the Nagare Boma, Yamimaru and Kirika) who pretty much were the link between the high school setting and the demon villains. In my opinion, Kirika is the character that used the most effectively the concept of "sympathetic villainess"; at the beginning, we see her as a pretty sympathetic character, who's forced to discover her real background. As a result, she becomes a villainess because she felt it was what she had to do. But it's obvious that she's not really evil, and when she comes in terms with her origins, she doesn't feel the need to do evil stuff and fight, and comes back being what she really was: a nice girl.
However, Fiveman did introduce some new concepts. It was the first sibling sentai team, a concept that is gonna be used in some following series, and once again the theme of family was important. Moreover, the a lot of wackiness seen in the daily life of Zone (the villain group) is a precursor of future 90' villains groups, like the Bandora gang or the Yokai of Kakuranger.
To conclude, in nine series, Soda brought a lot of new concepts in sentai, handling plenty of different themes, more than ever fleshing heroes and villains, and telling in each season a detailed story.
All those series have changed forever the sentai franchise, and influenced a lot future series.