We REALLY need an Ignore button for this forum.

Question: say we get a flashback where he learned what happened to Yamato's parents. What to former sentai actors would you cast as his folks? I'd pick Keiichi Wada and Sayuri authors. Both of them would be in their 40s now, I think.

Well, I think having 2 Former Sentai Actors to play Yamato's parents is make sense because Zyuohger is 40th Sentai Anniversary (I'M NOT COUNTING NINNINGER AND EVERYONE SHOULDN'T COUNT THAT SHOW AS ANNIVERSARY SHOW!)

But to be honest, I am more interested of what is the argument with his parents that made Yamato run away than who playing them.

Oh damn, my phone autocorrected "Uchida" to "authors." Whoops.

Uchida probably won't happen since she left showbiz.
Wada is a possibility since he makes his rounds for toku stuff.
Personally would like to see Mr. Gao Yellow aka Horie.

Why we don't have more green sixth members ?

Luckily for you, they're counting Zyuohger as the 40th anniversary, even though technically Ninninger was the show airing on the 40th anniversary of the first episode of Goranger.

Anyone else notice that when watching an animal-motifed Sentai where the visors are an animal's mouth that you tend to look at the helmet's animal face as the Senshi's face and that their talking out of their eyes?

Variety.

For myself some do some dont.
Stuff like Gaoranger where the motif is quite emphasized do whereas Zyuranger or Dairanger I see the visor as visors.

Since Ninninger had guest appearances from some of the ninja themed previous teams, I am expecting at least member(s) of Gingaman and Gaoranger to appear. After that, my best guess would be other sentai with animal names but maybe not so strong on the theme like Sun Vulcan, Liveman and Jetman. Any others care to guess?

A bit of a problem is that, of your list, only Gao and Ginga actually made use of the animals. The others just had animal themed costumes.
How about Go-Onger, though? The Engine Souls were fully sentient, after all~

But I'd totally be down for an animal Sentai team up akin to Kyoryuger's dino Sentai team up and the NinjaRed and HurricaneRed cameos in Ninninger.
Hell, bring in Green Sai or GekiChopper just for Zyuoh The World~