I'm of two minds on it...

FOR Derplander, it would just be kinda fun to have him there...maybe see how he because the WoD, instead of (a) WoL. Was he there for Legacy? Did he fail at something? Maybe he was never sent forward in time...or maybe he was, but to somewhere completely different? Maybe the whole Dalamud dropping with nothing he could do broke his belief in Hydaelyn?

FOR Doppleganger: We already know that the FF universe's are connected via the Void (Since thats how Gilgamesh gets everywhere)...but what if it isn't just different FF universe, but multiple versions of each universe too? The WoD Doppleganger isn't just some clone, but an actual alternate universe version of us that for some reason went to Zodiark instead of Hydaelyn. As well for this version...while for many it won't matter since their main has always been the same, I think it would be kinda nice if they stayed the class/job you started out as EG If you are a Paladin who eventually decided to be a Dark Knight, the doppleganger would still be a Paladin type of thing.