Fetches will not be in KoT. See here: http://www.mtgsalvation.com/forums/the-game/modern/554589-aaron-forsythe-no-fetchlands-anytime-soon
Basically, Magic R&D chief Aaron Forsythe said they want to reprint them, but the urgency of the reprint only became apparent recently, and there is a long turnaround time. I mean, MaRo said recently on his blog that design (though not development) is over on "blood", the set that will be released in fall 2015. That's 15 months away.
In fact, that's my guess as to when we'll see a fetchland reprint. It fits the time frame, and they won't take 10 land slots in a core set (that's a lot of space). They're also not very enticing to new players (core sets' target audience) because they cost life to play, and they won't see the interaction with shocks/ABUR duals, they would be disappointing to them.
Another thing: they will reprint all 10 fetches. MaRo has said repeatedly that they now believe that printing fixing for all 10 color pairs equally is better for the game then only priting half.
And it won't break modern. People believe it would break legacy when they printed the enemy fetches, but literally nothing changed. Jund will still generally need to lava axe itself to thoughtseize yourself on turn one.