Fetch lands!  This alone makes the set exciting.  Then more mana fixing in the form of the wedge lands, and then more mana fixing in the form of the paired coloured lands.  I'm going to have to do a lot of thinking as to what I'm going to take out and add in terms of land.  And the whole set isn't even released or fully spoiled yet!

Anyways on to a more organized breakdown of how this set will impact my cube.

Fetch Lands
I am excited to see the allied fetch lands like Bloodstained Mire make a return.  Only because they will hopefully be more affordable.  Extra mana fixing is always nice as I have a large gold section (about 25%).  If I obtain a set, what do I take out?  My initial thoughts are Rupture Spire and Transguild Promenade along with Terramorphic Expanse, Evolving Wilds and Temple of the False God.

Wedge Lands
These are like the shard lands of Alara (i.e. Crumbling Necropolis) but produce the wedge combination of colours (GUR, RWB, GBW, UGB, and URW).  I'll probably take out the vivid lands (i.e. Vivid Grove) and replace them with these.  However, if I don't get a set of fetch lands, I'd take out the cards I outlined above instead of the vivid lands.

Tarkir Refuges
Couldn't think of a better name but there is a cycle of 10 lands that is functionally identical to the Zendikar refuges (i.e. Graypelt Refuge).  The entire cycle of allied and enemy pairs is in the set under different names.  I think I'll replace the refuges in my current cube with these new ones as well as the enemy coloured cluestones/keyrunes from the artifacts.  This will shrink my artifacts in the cube a bit but doing so would allow me to remove any artifact specific hate cards and replace them with better ones.
Why not just replace the guildgates?  By not replacing the guildgates I give myself four full sets of lands that produce both allied pair mana and enemy pair mana (the Tarkir refuges, the guildgates, the check lands and the scry lands).  This is better overall especially considering the large gold section of my cube.

The Banners
These are like the cluestones but produce the wedge colours.  If I take out the enemy cluestones/keyrunes as per above, I'll replace the allied cluestones/keyrunes with the banners.  The mana cost is identical but can produce more types mana and the banner is always useful as you can cycle it away later (unlike some of the keyrunes which lose their usefulness at some point)

The Ascendancy enchantments
I had to pick a cycle from Khans of Tarkir to put in my cube, this would be it.  They provide a decent effect and don't cost much to cast.  However, I'm quite happy with my gold section thus far so might not try too hard to fit them in.  If anything I might try to replace one of each shard card (I have two cards from each shard) but that would mean I can't have Sharuum the Hegemon and Sphinx of the Steel Wind in at the same time.  And those two are so nice together in a reanimator style deck.

I haven't seen much else I'd like to add (except for maybe some of the mythic creatures like Hooded Hydra or Wingmate Roc) but only about half of the set has been spoiled (see here) so there might still be some more interesting things to see.  Hopefully I'll be able to play in the pre-release next weekend.