I think every weapon should have 1 upgrade slot, it is entirely up to the player what they choose to put in this slot. The catch is that it has to come from a weapon that has been broken down, you basically get to choose one perk or a modifier to keep from that weapon type. For example, if you wanted the solar damage type to occupy your upgrade slot on Suros Regime, you'd have to break down Abyss Defiant in order for that to be possible. Want Firefly on your Timur's Lash? You'd have to break down a Fatebringer etc. What you can't do though is take Firefly from the Fatebringer and put it on an Auto Rifle, it would have to be from the same weapon class. You also wouldn't be able to break down an exotic to gain a perk to put on another weapon. So why not just be able to choose from a list of upgrades instead of having to break down a really good weapon? Two reasons for this, the first being that it should be a meaningful decision on what upgrade you want to put in the weapon, some weapons with unique perks (VoG weapons) can take a bit of time to obtain. So it could be a tough decision on whether you want to transfer that perk over. The second reason is that it would revitalise old content, more reasons to run old raids is always a good thing. That said, some perks could make the weapons vastly overpowered, whether they should be limited to their original weapon remains to be seen. White Nail on the Black Hammer as an example, being able to put this on the Ice Breaker would be somewhat broken. Even if a few limitations were in place it would still allow players to make some pretty unique weapon variations.