Price reduction, probably. Think $100-$200. Better battery life, up to 10%-ish. Butterfly keyboard, doubtful but who knows. Most (from what I can tell and personal experience) are pretty happy with it now. Don't expect the things people bitch about though, no changes in ports or suddenly able to run like an iMac or return of a glowing logo or MBA battery life or anything else significantly different. Not an insider, just a fortune teller like the rest, based on historical Mac updates.
As far as holding on to your horrible PC, depends on your tolerance and how much crap you need to port over. I can see that changing mid-year would be inconvenient for a student, but maybe your contents are pretty light. 15" and 13" are two different animals. I would never buy a 15 because I usually dock at my desk. 13 form factor is for travel and mobility. It would be different if it was my only computer and I didn't have an external monitor, then it would be 15. We all have opinions about 2015 vs. 2016 but either will work for the next 3 years. If you want to go beyond, I personally think the TB3 is going to be a big deal, but who knows right?