I think the whole "Each mech chosen represents the main components of the most common themes" idea also works with some of the other ideas.

Daizyujin is the first dinosaur-based mech, as well as representing Power Rangers.

GaoKing is the 25th anniversary sentai's mech and also best represents using animals as mech components.

DaiBouken was from the 30th anniversary and best represents using vehicles.

MajiKing is a fan favorite and is also the first and only mech to use magical components.

Combining them all possible reasons just makes them all make more sense as a whole.

All of those mechs are my childhood, minus daizyujin of course, but if thats what this show is going for, and a heavy dash of ranger key esque legacy based collectibles, hoo boy my wallet will be crying. Gokaiger did a number on me and a series that follows a similar modus would definitely get my money. I liked kirameigers designs, but the whole adult acting like a child route that sentai has taken in recent years is really off putting, to me personally. If sentai got back even a smidge of that swash buckling brovado that they had pre- kyoryuger, i would be sooo in.

I would probably argue that the stuff from Dairanger would count as magical creatures as well- but between it and Magiranger well I'm pretty sure Magiranger- and thus Magiking is a better category mascot.

Dairanger, Ohranger, Gingaman, Magiranger, and (to a certain extent) Goseiger, Ninninger, & Kyuranger all have magical beast-themed mecha.

The only correlation I see is that Daizyujin and Daibouken start with "Dai" which means "Big/Great", and Gaoking and Magiking both end with the word King. Other than that I guess we will have to wait and see what their justification is. One thing for sure, the gears are the rangers keys of the show and instead of cosplaying as senpai rangers they will harness the power of the 45 mechanical wonders(let us not forget LuPat which gave us two of 'em) that roughly costs about 70 bucks USD at your local Japanese Toys”R”Us.

If these rumours are true, I figure the main reasons will have been marketing/iconic-ness. I figure it's more likely that they thought "what are four well-known robos we can use, that are all of vaguely different styles" rather than "how can we categorise every season's robot theme", if that makes any sense.

That said though, if this is true what are people thinking Zenkai-Oh might end up like? I can imagine getting "new" versions of each of those series' zords, the same way Machalcon in Gokaiger was like a "new" Go-Onger zord. It would be kind of interesting to have five zords of different design philosophies form one coherent robo, especially given how aesthetically different those four are!

I think people forget that Daizyujin means more than just 'power rangers' -- Zyuranger was an incredibly popular season in its own right, and between being the first to really be a fantasy story instead of sci-fi and the first to really have a sixth ranger beyond one-offs, it was a big landmark series for Sentai; all of which matters a lot more than what overseas did with it.

The question is will kiramager end early with the show seems like speeding up like they're in the 40s plus we did minus 4-5 episodes due to covid breaks and other events. Unless they're delays in shooting the next ones, then we get extended kiramager or mini specials like in old days or during lupin ryusoul transfer .

Remember, Sentai debuts in March now due to Sentai Strongest. So there's three months of episodes left; meaning it'll likely end around 45.

Newest speculation/rumor from Matt Hunt/Dukemon on Twitter:

Some of the Zenkaiger mechs may have a KR W-like half & half combo gimmick.

Seems believable, but as he likes to say: take this with a grain of salt.