Im guessing that you will need something only attainable through the new arena to upgrade things like vog armor and weapons. Just like you need the crux to upgrade the husk of the pit to a legendary, or whatever that weapon is called, you will need whatever the new equivalent of the crux is to upgrade those things.
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Exactly what I was thinking. Just because we'll be able to upgrade doesn't mean it's not going to require doing the new content (Osiris/Elders).