Just in case there's anyone who hasn't seen these images everywhere over the last couple of days.
Zenkaiser's real name is Kaito Goshikida. His parents were scientists who disappeared shortly after discovering the existence of parallel worlds, so he now lives with his grandmother who runs a sweet shop / cafe. The only reminder he has of his parents is a mechanical bird called Secchan (which appears to have a lot of detail based on Variblune from Goranger, to the point where people have speculated whether it will transform).
The mechs are called Kikainoids. They come from another dimension called Kaikaitopia, which has been taken over by the Evil Dynasty Tojitendo. The Tojitendo have adapted technology developed by Kaito's parents to create the 'Tojiru Gears' which they use to "lock away" a parallel world. This then allows them to create a monster based upon that world.
Kaito's parents also created the Geartlinger, a gun that utilities the "Sentai Gears". Kaito uses this to transform and to make his Kikainoid friends grow large, and can also use it to summon the powers of previous Sentai.
The four Kikainoids are:
Zyuran - the "old man" of the team, based upon DaiZyuJin from Zyuranger.
Gaon - the quiet, standoffish member, based upon GaoKing from Gaoranger.
Vroom - the serious yet curious member, based upon DaiBiouken from Boukenger
Magine - the shy magic/occult nerd, based upon MagiKing from Magiranger.
So it looks like the four Kikainoids will have a slimmer powered down mode that will be used during the "untransformed" scenes, and then when Zenkaiser transforms they will turn into their bulkier forms. They can then grow and change into an alt mode or combine.
These slimmed down forms take cues from the costumes of the series that inspired them. So Zyuran seems to wear shorts similar to the pants worn by Tyrannoranger, Vroom seems to have a colour scheme inspired by Bouken Blue's jacket, and so on.
The bulkier forms seem to be called Zenkai, which is added to their names. So they become known as Zenkai Zyuran, etc. It also seems that in this mode they have a number on their foreheads corresponding to the number of the season that inspired them.
Each forms half of a combined robot - yellow and pink are right sides while red and blue are left sides. It seems likely they will be sold as a yellow/red pair called 'ZenkaiOh ZyuraGaon' and a pink/blue pair called 'ZenkaiOh VrooMagin', but you can of course switch them around if you buy both and create 'ZenkaiOh ZyuraMagin' and 'ZenkaiOh VrooGaon'.
The official toy reveal will apparently happen on December 26th but cast reveals won't happen until January.
The series will get an episode 0 as part of the "Super Sentai Movie Ranger" triple bill and then episode one will broadcast on March 7th 2021.