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You are right that Sisters need more Superiors, but my issue with that model is that they are essentially sergeants and as such they're not supposed to be much different from basic troops; and I'd rather avoid copying the standard sororitas model (it may be extremely tedious to try to replicate the detail of the original armor).
The Hospitaller, Imagifier, and Dialogus are already fantastic figures so anything I could model will pale in comparison.

I will consider the Imperial Guard riders if there are no other suggestions.

I know this is old but can you still find this stl somewhere, the link from the OP doesnt seem to work anymore. Really cool looking model