And a netherworld definitely doesn't look like an Arena.
Also, it would be a sucky game if you know you're dead with no way of return. Bit of anti-climactic.
I think its either Vader like you said yourself.
However, it wouldn't explain why he's fighting in an (not Imperial controlled) Arena. Every Jedi would be shot on sight under Palpatine's or Vader's command. And since it's obviously a continuation of the LS ending, Marek IS a Jedi.
I hope it's set at 12 aby or something. With 14-15 years passed since the last one. Marek would be early 30's and Luke would be searching every corner of the galaxy for Jedi students after the 'second' death of Palpatine. Maybe Marek can help Luke before Luke starts training Corran Horn and Kyle Katarn? Marek can die/disappear before that and re-emerge in the Vong War for a TFU3.
How else can they entwine this into the already crowded EU from 20bby-0bby?
"I think the Devil ****s Dutchmen."