I agree, thematically she had to die. Without an onscreen death there is no closure for her character, and considering she was a major player in the first three episodes, having her just "fade away" between trilogies would be weak to say the least. I understand Lucas' need to have SOME mention of her in the OT, but it was too little, too late, too...confusing. The absence of any references to Padme in the OT aside from Leia's short remembrance are now a glaring continuity error. They weren't back in the 80s, as Anakin's wife was just the dead mom nobody talked about; fair enough. The current situation with Padme is comparable to if Han and Leia had gotten married and had children n RotJ, but Leia died in childbirth at the end of the film, only to be completely forgotten in Episodes 7, 8 , and 9. Lucas really should have fleshed out the backstory and formed some more concrete plotpoints for Mrs. Skywalker back during the OT days.

Things being as they are, the best way to have handled this in RotS would be to have Leia born first, Padme hold her for a while, make eye contact, name her, and kiss her. Then she would go into a new set of contractions as the unexpected Luke arrives. She'd name him, then say "There's good in him; I know there is still," and die before getting to hold or look at Luke. This is what I think we were all expecting to happen; of course, that's probably why it didn't.