Taking this question from another perspective:
[Out-of-universe] Hugo Weaving was not willing to take back his role as Red Skull
[In-universe] and it seems that any generic ominous figure with a black cloak dubbed by Max von Sydow would have done the same job perfectly well
Why was it necessary to have Red Skull as the guardian of the Soul Stone? Is there something in Captain America that explains this continuity? Or some hint to future movies? Or is it just to reinforce continuity generally speaking?