I think I have to agree that showing an amalgamation would only be confusing if we have never seen him that way.
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Alternatively, if you tried to depict a Hayden-Shaw amalgamation, or a very old Hayden, I think it would work better to change Shaw’s appearance in ROTJ to establish that look and for it to be not be confusing.
It might be difficult to craft the right amalgamation, but I think there is probably a perfect middle ground between an aged up Hayden (similar to Sade’s example) and whatever sources we can find for an age appropriate Shaw. I don’t think it would be confusing as long as it looks sufficiently similar to Hayden for the audience, and sufficiently similar to unmasked Shaw to provide visual consistency from Luke’s perspective.
I think I’m going to try and start looking for some high quality Sebastian Shaw sources to pull from and see if I can mock something up myself. Man do I love this forum.