It looks like you are wanting a Shimano freehub that takes a six bolt brake rotor. You have few choices. You could get another M525 or the M756. There are very few differences between those two. Of special note, the Body Fixing Bolt is the same in both.

You might want to check the M529 hub. That's the "stronger" "29er specific" six bolt rotor compatible hub. It isn't clear what's stronger in the 29er hubs. One might presume that the freehub bodies are different, but no, Shimano uses the same part in 29er and non-29er hubs. I suspect that the difference is in the actual hub shell.