In long:
I've recently taken to wearing a new type of outfit to work:
  • Jeans or casual trousers
  • Shirts (various colours and patterns), unbuttoned
  • Sometimes a tie, with a loosened knot
  • Waistcoat (various colours: black/grey/blue), always
This has been working well, and I've no plans to change this style - the question is about wearing something over it, on the way to work. Over the winter I've been wearing a light overcoat in grey, and that hasn't clashed with anything I've worn under it. But as the weather is getting warmer, I'll be packing the winter coat away and so I need something different.
I had been thinking of a jacket or blazer, but when I went through the couple of suit jackets in my wardrobe (black, charcoal) there was an obvious problem: a formal jacket really does not go with a formal waistcoat of a different colour. The only combination which worked at all was a blue moleskin blazer over a faun cotton waistcoat - which I think was OK due to the big contrast of colours and the informality of both the items.

I had been thinking of a blue cotton blazer perhaps - but now I'm not sure. Could any smart jacket or blazer go well with a variety of formal waistcoats worn informally? Or do I need to change direction and consider something more deliberately informal like a grey tweed? Of course one problem with that is that I couldn't wear a tweed through the summer (I'd boil); also I just don't like tweed very much.