Borderlands 3 does already have a lot of good and/or interesting items that returned from previous games but I think there are a few nice ones that we are missing so far:
The Longnail/Amigo Sincero: A sniper rifle with the ability to ignore shields. The Longnail is Vladof and from the Pre-Sequel and the Amigo is Jakobs and from the most recent DLC for Borderlands 2. There was also a sniper called the Tresspasser in the Bl2 Basegame but it was poorly balanced, with underwhelming damage and outright horrible rate of fire (it had niche applications against 2 raid bosses but that was about it)
The Chain Lightning: The Chain Lightning is a legendary grenade from the Dragon Keep DLC of Borderlands 2 (another grenade from that DLC that made it into Bl3 is the Firestorm). The Chain Lightning fired a more or less straight lightning strike that would ricochet off surfaces and enemies to the next enemy. It also passively regenerated grenade ammo (so did the Firestorm but sadly they don’t seem to have implemented that).
The Bessie: A pearlescent Jakobs sniper from Borderlands 1. 100% accuracy, very high damage, 500% critical damage but in exchange a very low rate of fire and magazine size. Essentially the kind of stats a sniper would need to remain viable in M4 without overkill perks.
The Fibber: An interesting pistol that had three different variants, with one firing off very slow spread shot rounds, one firing a slow and heavy projectile with very high crit damage and the last one firing a normal bullet that ricochets into a shotgun spread. The Fibber also featured a wrong and partially ridiculous stats (+50% love) on the item card.
The Chopper: An Assault Rifle that fires several bullets with each shot at a very high rate. The Mag size is equivalent to the maximum amount of AR ammo you can have with all SDU’s (if they were to make it COV it shouldn’t overheat). The payoff consists in the weapon continuing to fire after you pull the trigger until it is interrupted, either by a melee attack or swapping to another weapon.
The Evolution: An adaptive shield, meaning it provides you with elemental resistence to the last element that damaged you. The Evolution features particularily high resistence up to 86%. It also features health regen that increases the lower your shields are and increases your max health in exchange for a relatively small capacity.
The Sham: A shield with an extremely high chance of absorbing bullets (up to 94% if you get perfect parts). You do however get a small overall capacity and slow recharge rate.
The Pitchfork: A legendary Dahl sniper that fires 5 shots in a horizontal spread at the cost of 2 ammo.
The Lascaux: A non-elemental Dahl SMG that fires 2 bullets at the cost of 1. When shot at a surface, the bullet decals will recreate one of the Lascaux cave paintings.
The 12 Pounder: A torgue rocket launcher that fires a single canon ball at high speeds. Very high damage. The canon ball ricochets off surfaces and explodes on impact with enemies.
The Skullmasher: Essentially what you would get if the Maggie was a Jakobs Sniper Rifle. It shoots 5 bullets at the cost of 1 ammo, with the individual bullets having lowered damage but the overall damage being very high.