The Cubs won 73 games last year.
I don't think it's total fantasy to think they can up that to 86 or 88. I think, obviously, 81-84 is more realistic. But is 86 or 88 out of reach?
They improved 7 games from 2013 to 2014 - what did they add? Arrieta for 2/3 of a season? Hendricks for 1/2? Don't forget they also lost Shark for 1/2, and didn't win his very winnable starts anyway, due to offense and bullpen, both of which are improved. And they'll add 1/2 a season of Hendricks again. And 1/3rd of Arrieta. And some other guy they picked up.
What are the odds a team could go from 73 wins one year to a pennant the next?
Well, 3 times in just the past ten years teams have gone from fewer than 73 to the pennant. And we're not even talking winning the pennant - we're talking contending for the playoffs.
It's not fantasy. It will take things going right in a lot of areas, and they don't have much room for error. Injuries could easily derail them. (Then again, they lost each of their two best players for a month last year due to injury.)
Contending in 2015 is very realistic.