These could be as simple as a character or a MacGuffin that seems to be forgotten, or as complicated as the theme and plot of a whole forgotten series. A quick list of ones that have come to mind for me lately:
1) Dr. Carolina Washington and her method (used in the HAMMER storyline where an escaped Norman Osborn united HAMMER, Hydra, and AIM under one banner) to take genetic samples from superhumans as diverse as the Hulk, Spider-Man, and Giant-Man to give similar powers to any number of agents, such as the Hulked-Out Hydra Agents, or even mix and match, as with the Spider-Hulks. This needn't be as a member of an 'evil' organization such as those... she could easily have cut a deal with SHIELD, the US government or military, or some other legitimate government to share her techniques, or even have served any sentence imposed for her time with HAMMER and now be going into business selling superpowers to whoever can pay.
2) The power disruptor device initially developed as Stark Tech and then rebuilt by Jonas Harrow, which can be used in the field (as opposed to in some superprison) to shut off power sets as diverse as those of Ares and Clint Barton (perhaps from residual Pym Particles from his time as Goliath?) while causing a lot of pain and shock to those affected, but be tuned or blocked in some way so that others in the area of effect such as the Wrecking Crew remain unaffected. This could be a great equalizer and force multiplier for some organization such as SHIELD or the NYPD, or even for individuals such as Deadpool or the Punisher, and once developed and used one would expect a lot of attempts to duplicate or recreate it.
3) Diverse groups such as the Live Wires and the True Believers, and seeing what they have been up to while invisible for the past umpteen events.
Curious as to what everyone else's picks would be in this regard.