Now that some new stuff has already been teased for helping to create more differences between Iron Hills and the two other realms, perhaps it is time to reconsider the topic.
Tienety got the right idea I believe: Iron Hills Stonhelm could be more aggressive than Erebor's as he is younger and then less wise. That's a start. Let's not forget his role is to tank and there must be strong connections between both skillsets. Pretty sure we can get to something.
The rune powers are very fine (units related for Iron Hills, buildings for Erebor), no change needed.
Dain's Heritage fits well Iron Hills. Erebor counterpart could be passive experience boost. Upon activaction, Stonehelm gains extra experience and armor while fighting, but the passive leadership disappears until cooldown is off (same way). "Prince under the Mountain" is a nice name (Tienety).
Righteous Heap is more suitable for Erebor. With Iron Hills, Thorin could gain extra damage or speed instead of armor.
Iron Determination is a though choice, but I'd go for Iron Hills and give Erebor Stonehelm a new unique level 10 ability: Thorin Stonehelm gains X% additional armor for each ally hero around him. This bonus also applies to other heroes.
He could also keep the regular passive ability "keeps fighting when his health bar is empty".
I think the last spell could be a unique addition to the mod and for Thorin's tank role. Plus as Adamin said, it reminds a bit of King Dain who fought side by side with Brand of Dale, but there is still plenty of difference between Stonhelm and his father. As for the design, I trust you guys, if you ever think he needs a new one. I'll just say that I like the axe in the first post.
