No any critical flaws found in Nxt-like proof-of-stake. On other hand, no any strict formalization made yet as well.

So there are two things to be done:

1. Formalized model showing Nakamoto's property could be met in proof-of-stake with contribution to multiple forks allowed(in other cases there are other problems with formalization). Simulations show the property is seems to be met, thanks to cumulative difficulty working more or less ok as fork selector function(btw, PoS coins with longest chain rule have problems here, at least).
I'm now talking with guys much more skilled in CS/math about possibility of the truly formal framework.

2. Practical contributions to Nxt / other projects around. Nxt's algo seems to be pretty safe, though block delays distribution is needed to be better(closer to average value). It will reduce or mb even eliminate incentive to contribute to multiple forks(by trying to do private branch attacks, then share private forks, then we have majority of forgers having multiple-branch forging with N@S possible as result in such environment). I hope some improvements will be made in 1.7/1.8.

And yeah, "test coin"(don't like "coin" word here, I would like to call it "experimental blockchain engine for hackers"). Making some changes now, so it will be possible to switch Qora's PoS to Nxt's by changing 1 line of code(and introduce other consensus models easily). Then yeah, multiple branching will be introduced in Scorex. Some non-consensus things will be tested as well, e.g. Bill White's scalability proposal etc.