This is an interesting concept.
I remember that it was also suggested on the podcast the Bohrock-mechs/Boxxers' return as Matoran vehicles during the War, so it would stay in theme.
The boxxer would make for a great $12 hero battle pack, a boxxer and 2 matoran and some enemy minifig, similar to the nexo knights equivalent,It could even be based on the recent small mech torso Lego made, using mixel joints.
I'd imagine it would be like the star wars/nexo knight build-your-army sets.
(I remember, the concept was a mining bot repurposed for war, which I really like)
Why not make any number of toa combine?
This opens doors for any number of team combinations. Story wise, two toa kaitas could be commonplace to overcome various obstacles, such as taming the forces of nature. Three toa kaitas could act in a manner similar to megazords from power rangers: used to kill the bad guy of the week/story arc. Four and five toa kaitas would be reserved for the greatest of threats like an army of manas. Finally the toa nui would be the last resort when fighting Makuta himself.
Just a thought
When you say 3+ Toa Kaita, does that imply 3 Toa per Kaita for 9+ Toa? There is only 6 Toa, so 3 Kaita would be 2 Toa each, and 4 or 5 Kaita couldn't be done.
I assume you meant to say "Toa" instead. (3 Toa combine, 5 Toa combine, etc.)
I meant kaitas made of 2-6 toa. I guess I should have said two-toa kaita as in kaitas made of two toa, three-toa kaita made of three, etc