What you do with a toy plane, swoosh it, kids like to swoosh their action toys. Therefore they need swooshability.
Honestly I don't even remember the set line up of the first year.
The sets were sold as good guy sets, I remember that much.
So have the minifig toa fight brick built rahi for the first year, most likely mounted by the brotherhood.
Then the rahkshi, I'd argue they shouldn't be minifig sized, after massive brick built rahi it feels like a significant downgrade, as well, I doubt they'll be riding anything, so action set potential is kinda low with the main villains being minifigs, and, how will the toa defeat the rahkshi? I argue an upgrade of some sort, I'd have the toa find the kaita machines, for the sake of having sets in the second year, either the we get brick built rahkshi, more rahi and/or we get the kaita mechs.
I'm sure you're fine with structures, but kids aren't, you need some sets kids can swoosh along with the scenery.