Darker And Edger VS. Lighter And Softer Heisei Era Rider
A bit of a history lesson would take us to how Kamen Rider was cancelled after Kamen Rider Black RX. So no new series came except for a few short movies. Two years after the death of Shotaro Ishinomori the Kamen Rider franchise was revived with a new format. Kamen Rider Kuuga to Kamen Rider Kabuto are considerably that violent and scary. I call that era as the "Nightmare Era" of Kamen Rider.
The nightmare era of Heisei Rider
This era was already in conflict with the tone of Super Sentai after Timeranger. Post-Timeranger Super Sentai tend to be much less serious no matter how good the story is. Gaoranger was more of a fun ride with much less nightmare fuel than its predecessor Timeranger. But the year that Kuuga aired was also the year of Timeranger. Both series are considerably darker and edgier. Although Naruhisa Arakawa is more known for preferring lighter and softer but he did a good job with Kuuga. Toshiki Inoue was also involved with some Kuuga episodes though I wish he were involved with Timeranger instead.
What these shows had in common were pretty much a lot of scary monsters, deaths and a lot of stuff I enjoyed about some of the older Super Sentai series. Kuuga alone had such a huge body count with blood all over the series. After the success of Kuuga we've had Kamen Rider Agito which was an atypical Kamen Rider series. Agito had a lot of mysterious deaths and really frightening scenes. Maybe you've had people getting scared of monsters suddenly killing them in the most bizarre ways. Both Kuuga and Agito were pretty much the start of the nightmare fuel era. There wasn't much of a contrast between Inoue and Arakawa at this point. I personally think both series should have been kept connected. Just because one hero succeeds doesn't mean evil is vanquished forever. There's still a lot more threats and Yuusuke should realize that there's more than just him to protect the smiles of everyone.
Kamen Rider Ryuki followed just year after Agito with both darker and edgier writers Inoue and Yasuko Kobayashi. It was said they had a writer war but it's more like a writer vs. producer war during the whole Rider War. The series involved a lot of really unlikable characters and Shinji the main protagonist is a huge motherf*cker. Then we had the first really evil Kamen Rider with Takeshi Asakura who became Kamen Rider Ouja. Ouja was pretty much Bilgenia with the King Stone! A lot of plots in Ryuki progressed from dark to darker. Then the real darkest plot ever came: EXECUTIVES! Yup, they made the whole show non-existent by the power of executive meddling!
Perhaps the darkest (but also the most mixed) one was Kamen Rider Faiz. Faiz is just too depressing and it takes the whole mutant/human war to a whole new level. If Ransik from Power Rangers Time Force saw Kamen Rider Faiz or heard of it he'd probably say, "So that show takes the whole mutant/human war to a whole new level? Really? Tell me about it!" Kamen Rider Blade dealt with the supernatural once more with the Undead. Kamen Rider Hibiki had really slow first half (no thanks to Tyuyoshi Kida and I honestly prefer Inoue's turn) but it had some nightmare fuel. Kamen Rider Kabuto was almost a tribute to the Aliens franchise in some way.
The lighter and softer era of Heisei Rider
Even before the post-Decade era, Kamen Rider started to get lighter and softer which I think can be considered a breath of fresh air. While I do prefer serious shows but it's also nice to have less serious shows. Just think after having God Fire Tonkotsu I'd like to settle for non-spicy noodles anytime. So what went on was basically trying new stuff.
Kamen Rider Den-O and Kamen Rider Kiva is what you'd call Kobayashi and Inoue attempting to be funny. When it comes to comedy the two either hit or miss. Den-O may be lighthearted and you've got a loser turned into the hero type of story. Kiva is more of comedy than horror with the Fangires. Horror-comedy has always been part of the many innovative ideas and I guess Inoue wanted to attempt it but he was burnt out during Kiva. Kobayashi was trying to do something lighter than Timeranger but may have ended up creating a confusing loop. Maybe Kobayashi was trying to create a 50 episode version of Changerion (ex. both losers who become the heroes) with a really confusing ending. I do have a soft spot for Kiva though and maybe I'll develop a soft spot for ToQGer as well or maybe I do but I just don't want to admit it.
Kamen Rider Decade was well theoretically pretty serious with the destruction of worlds but ends up as a confusing mess. While it's aesthetically better than Saban's Mashed Rider but the writing quality tends to go down. I viewed it as the "superior sequel" because it did a lot of things right over Saban's Mashed Rider like better action, better acting, better special effects and the crossover with Shinkenger beats "A Friend in Need" by several miles. But I can't deny how I can't warm up with the show overall. Good thing Gokaiger fixed this mess and produced an anniversary season that Decade never was!
Post-Decade shows were pretty much lighter and softer. Kamen Rider W may be a detective show but it's mostly lighthearted. No super dramatic moments like in the Rescue Police Tokusatsu like Winspector, Solbrain, Exceedraft and Janperson but it's still a good show. Then we had Kamen Rider OOO which was yet again Kobayashi's attempt to write lighter and softer. Good thing Kobayashi was able to pull through it with Shinkenger but she just isn't that good with lighter and softer stuff. But still it'd be good for a learning experience to try different stuff right? Kamen Rider Drive followed as another detective style Kamen Rider. I personally prefer Drive over W though Riku Sanjo was also responsible for a series I personally don't like for its plot (but love the action scenes) namely Kyoryuger. Both W and Drive are pretty good series to me.
Kamen Rider Fourze, Kamen Rider Wizard and Kamen Rider Ghost is what I'd call another attempt to merge Nickelodeon with Kamen Rider. Kamen Rider Wizard had an interesting concept and better characters than Magiranger (no HOUKA to ruin the show) but man Kida makes it so boring! Fourze was pretty much a fun ride attempting to make a few characters in Jimmy Neutron look like they've all grown up and something that puts Planet Sheen to shame. Kamen Rider Ghost entered in as what I'd call a Danny Phantom like Rider though I prefer Kamen Rider Kiva. Ghost had interesting historical tidbits but I'd rather be watching some documentaries.
Kamen Rider Gaim and Kamen Rider EX-AID are both continuations of the Rider Wars in some way. Gaim was pretty much a better written and better executed series than Ryuki. While Ryuki had cooler suits and power ups but Gaim succeeded where the former failed. Kamen Rider EX-AID is in some way another Rider War. It's not exactly darker and edgier but it still has some serious moments like it turns out Hoji Emu does have the Game Sickness (I might write on that soon).
In the end, it's never been a question of the mood but a question of how interesting it gets. You can be so serious and yet be so boring while you can be so lightherated and yet be so entertaining.