Just updating this thread with my current thinking on seasonal collections, which owes quite a bit to Ms Nix-Rice:
I now think seasonal collections should be seven-piece rather than six-piece. This allows for a better variety.
The collections come in sets:
Set 1: "Inner Column" -- a bottom and an underlayer top (or a dress but this is not quite as helpful usually). Usefully in a neutral (unless you love accent-coloured bottoms, and some do). A necklace and shoes in this colour are useful.
Set 2: "Outer Column" -- a bottom and an overlayer top. Usefully in the same or co-ordinating neutral (actually neutrals should all co-ordinate, right?). A necklace and shoes in this neutral, a scarf in both neutrals, a bag in one or the other or both neutrals are helpful.
Set 3: "Accent Pair" -- an over and underlayer top in the same/similar accent colours. Easy to switch in and out of the columns, or to replace the top.
Set 4: "Print link" -- only one item in this, some kind of versatile top in both or all three colours. Classic shirts can function as both under or overlayer, so they would be my go-to piece here. You could add a printed skirt to have an actual set, or replace the top with a skirt. Printed trousers? Not so much for me... Print dress? Possible, but again less useful.
Now for example I'm making my spring collection in white (neutral) and blue (accent), with a blue and white striped shirt. White skirt and trousers, white T-shirt and white linen shirt. Blue T-shirt and blue zip stretch-lace jacket. Jewellery pearls, crystals, silver and some blue and silver necklaces. White and blue scarf. Blue canvas shoes and light tan shoes (I don't do white shoes).
Could have done: White inner column (white trousers and T-shirt); Blue outer column (blue skirt and lace jacket); pink accent pair (pink T-shirt and hoodie coat); print link: pink/white/blue liberty cotton shirt. And that's part of the SWAP plan. This is easy to extend.