I only want to play Hidden Truths just to fully understand Anankos's backstory (and I did buy the DLC a while back). I've heard a few details about it here and there, but actually playing it would allow me to better comprehend those details. I should probably keep my expectations low going into it based on what I'm hearing though.
And yeah, I agree that Hans, Iago, and Garon felt shoved aside this time around. With Garon's case, it would've been perfect if we got to fight his monster form at some point, but nope, the only time in Revelation we learn about Garon being different from himself is when Leo and Xander notice he starts acting oddly, and then he appears just before the final fight, only to get devoured. Izana's death did feel unneeded; I didn't think he really had to kill himself just for the sake of getting Takumi to join Corrin's side. I mean, sure, Takumi was being skeptical, but that's his character, and he has every reason to act that way. As for the Corrin-becomes-king detail, Azura mentions right before the final cutscene that she handed the throne to him because she "never felt fit to be a ruler" when Corrin asks her why she didn't want to take-over the throne despite being next in line. For me, the part that feels a bit odd is that if you achieve S-Support with her, she technically rules Valla alongside you.
I too wish we learned more about Valla as a whole. After playing the game, I assumed Valla is supposed to be some sort of ancient kingdom because of how Anankos resided there for a long time, coupled with the curse so that people forgot it existed. I have a feeling the Bottomless Canyon could've been where Valla once was. Maybe once Anankos took control, he created a black hole beneath the kingdom, causing it to crumble from beneath and thus splitting it into multiple floating islands. Perhaps Valla was once located between Nohr and Hoshido since the Bottomless Canyon is located at the border. Then again, the game never talks about it. I also wish we learned about where in Valla does the crystal ball come from, and we would've learned that if the game focused more world-building on Valla itself. The only locations we see of Valla are the castle, a forest, and a few abandoned towns.
The transition between the moments after Anankos is defeated and Corrin is crowned as Valla's new ruler felt a bit odd too. When we see ceremony, we learn that Valla has suddenly returned back to normal; the throne room has been completely restored, and Corrin mentions that thanks to the generous gifts of land from Ryoma and Xander, "we've been able to rebuild the kingdom", not to mention you hear the entire crowd cheering for him. I guess Valla's broken-up land was also restored somehow, at least if the final cutscene is anything to go by. But still, was there a time skip between the moment everyone was like "Yay, we defeated Anankos" to the coronation scene? A little explanation would've been nice.