Milestones allow you to have a selection of the weapons from Virus and Zombie Massacre to use in the Plaza which is cool, but there’s still a lot of weapons from the two games that you can’t have at all. ZM would have a huge amount of milestones compared to everything else if all the weapons were milestones, though, so maybe they could be sold at the ZM upgrades shop instead (same with any Virus weapons at its shop).
It’d be fun to be able to use the weapons in the Plaza, it’d be a way for newer players to access guns without having to grind EXP, and assuming every weapon would have a price range like 10-50k units, it’d be a nice money sink for anyone with a ton of units on hand.