There are really dozens of ways upgrading Raid Weapons could be implemented in a reasonable way:
1.Upgrading the Raid weapons could reset its progress similar to old exotics
2.Upgrading a raid weapon could require a special item that only drops on VoG hard mode, similar how crux of crota will upgrade eidolon alley
3.Upgrading a raid weapon could require a special Raid bounty, like getting 500 Ultrakills with a VoC inside the Vault of Glass, or 500 Hobgoblinkills with A fatebringer, these bounties will drop randomly like the mentor missive from the bounty-bot, and only if you already posess the wepon needed for it
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I like #2 and 3!! :)
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I like #1 neither, but I don`t see a way around it, the way Bungie implemented upgrading up to this point. The upgraded VoG-Wepaons have to play really-hard-to-get, so that crota-weapons have at least this advantage over them;-)
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