Fashion is not only tight. It is soft as well. Most producers of suits and sport coats have minimized the amount of canvas and padding in theirs garments.
Cardigan style is fashionable
Not surprisingly the soft slim-fit fashion has paved a way for the cardigan. It feels like a sweatshirt or a jersey, yet the buttons on the front bring a touch of formality to it that can make it a substitute for a (soft) sports jacket.
Cardigan and tie don’t mix well
I would think twice about using a cardigan instead of a jacket though. The cardigan is a weekend and holiday garment, and I’m not fond of seeing it with a tie. I guess it can be done but in most cases I’ve come across tie and cardigan don’t harmonize. However, putting on a jacket above the cardigan can solve the conflict. Suddenly, the relationship works.
In general a neckerchief is better company for a cardigan than a necktie. For trousers grey flannel trousers are classics. You can wear corduroy trousers, chinos and jeans too. Shoes should not be formal. Chukka boots, loafers and sneakers are options, when you are outside.