great topic ele, i think waistcoats will be huge.. short tight versions, in velvet/jeans/mens fabrics (winter) or in linen/cotton for spring summer.
for me it's tight fitted mens-styled waistcoats over a low V neck sleeveless t-shirt and maybe a full over the knee skirt (a la Dries) or over a pleated skirt, truth i dont like too much fabric worn under my waistcoats.. i'm not for the bra option either
just something tiny and maybe fun or boyish like a mens 'beater?' (to avoid and twist the 'romantic/sexy/inviting' cliche) , a sporty t-shirt with 3/4 sleeves, a tight poplin shirt with rolled up sleeves to 3/4 or something completly transparent/lacey.
i like wearing waistcoats with both trousers and skirts, not sure if i'd wear them with jeans though, seems a tiny bit tricky and sorry to say...predictable.
*they also look good over oversized or empire style dresses.. both in fastened and unfastened versions