If I were in charge of M&S I'd:
Find a source of really good quality cotton and make some really good t-shirts that don't go out of shape or sag at the neck.
Do a range of plainish shirts in black and white and maybe a print or two - some in cotton and some in silk.
Make a range of these shirts which are a lovely cotton/silk mix, but in nice modern prints, and also plain.
Keep on selling jeans, as I think they do them pretty well, but sort out the black ones so they don't fade like buggery in the space of just a few washes.
Do a deal with a couple of classic brands (Levis maybe, Ralph L, Harris Tweed) and sell just a few of their pieces each season, which will become the basis for most of M&S 'fashion' advertising - brand name 'piece' paired with M&S basics.
Make far, far fewer items, but much better quality and fabrics. Pare everything back and produce an 'edit' each season.