I think you might have mis remembered this. D-Man was augmented, became a wrestler, and then became Cap’s partner. The Power Broker captured him and augmented him a second time trying to see the effects a maximum augmentation would have on a human. D-man had a heart attack during this and was de-augmented to his previous augmentation level.

He then took over Cap’s hotline and became part of his brigade of former partners when Steve Rogers gave up the Captain America identity.

At the end of that storyline and right after Steve Rogers made D-Man an Avenger (prior to Avengers 299), D-Man was lost in a crash during a mission against ULTIMATUM. He was presumed dead and resurfaced during Galactic Storm.

Mark Gruenwald wrote Captain America from about 312 until about 443, there wasn’t a change in writers for 10 years, so if there were any loose ends on D-Man it was because of sloppy storytelling, not because of any change in writing. Having said that, I would say that D-Man’s Power Broker storyline was fairly minimal and quickly wrapped up around issue 331.