You could take alot of "fun or fluffy" things with unbound IG, but after a few unbound trial runs there is a lot more ugly things with much more power with unbound formats in other armies or combos of armies.
I almost feel as if at this point I don't want to really play unbound armies as they can take all the strong units that break or stress the mechanics of the game to there advantage. Like tons of physkers for Deamonlogy or lots of cool imperial heros bro squads or scoring drop pods with powerful units in them... or empty to force you to displace crappy pods to get objectives.
It is just as possible to completely break Battle Forged lists as it is to break unbound lists. A friend of mine is going to play one of his buddies in another town in the most broken way he can think of. 6 detachments of Eldar, each detachment consisting of 2 Farseers and 2 squads of 3 bikes. That's 36 Warp charge dice before he even rolls the D6. What is making that so broken is that he is taking Daemonology on all of them and is bringing along 5000pts of Daemons. And with that many Warp Charges, he doesn't have to worry about the increase in perils.
So frankly, I don't think Unbound armies are going to be as nasty as people think. You get more flexibility, but can be out scored by your opponents troops, and people can get just as stupid with Battle Forged as they can without.