If you really want a phone you can root, I'm not sure that the GN3 will be the answer...I'd expect it to be just as locked down as the S4 is.
Also, as more time passes, the likelyhood of getting any device with MDK on it is probably getting slimmer.
Not trying to rain on anyone's parade, because I too find the ability to root and install custom ROMs important, but that is the reality of it. I'm lucky that I got my S4 and rooted it before ME7 came out...but I'm pretty sure my next device will be a Nexus.