If this hasn't been discussed yet, I'd beg to differ that during the final mission of the RoI campaign's final boss really is just SIVA "using the Iron Lord's armor". Going back to designs to the Remnants or "Revenants" as I like to call them, we can see ofc that they used a knight model and then morphed it into the figure for the boss. What keens my interest and to probably a lot of you back then, ghost doesn't describe the boss as, well, a revenant. Which, fact-wise, is another name/type of zombie. I know this may seem like it's something that shouldn't be put to much thought in. However though, this gives a possible plot twist that SIVA can "revive the dead". Unlike Taniks, which he was never living to begin with honestly, the Iron Lords' bodies were taken over by SIVA. Likely resulting in the Remnants' appearance and how the human body contained in all those SIVA wires/vines looks so mutated and slightly deceased. It's highly unlikely ofc that bungie will give any thought into this and use this as a DLC idea, but, there is room for an interesting story for this theory/idea