Massively agreeing with soft cuffed trousers or leggings. You do NOT want the bottom of your pants potentially trailing on the toilet floor. Trust us on this.
I travel short and very long haul a lot, both economy and business, depending on the reason for the flight. Often travelling from winter to summer or vice versa. I always wear the same thing, which is trousers as above, plus a soft t-shirt and a cardigan. It's all about easy-on easy -off layering.
I'm a great fan of a Uniqlo down jacket because they roll up small enough to fit in your carryon, so you can take on or off easily when you reach the outside world.
The internal temperature on the plane doesn't reflect summer or winter. It can get chilly or it be really hot. If you're on a budget flight which won't have blankets, take something like a pashmina or as someone said above, a poncho. Ponchos are fab for travelling.
Slip-on type trainers are great for flying. You quite often have to take your shoes off to go through security and they're quick and comfortable.
As you can tell, I do not care one tiny jot for style when flying. Comfort is everything.