For the mid-credit scene, Pip the Troll is a common recurring cosmic supporting character most known for being essentially Danny DeVito's character from It's Always Sunny in Philadelphia (there's a somewhat infamous issue of a comic where he temporarily has a cosmic cube and imagines all the things he might do with it, mostly turning every woman into a perfect 10 while making every guy a scrawny weakling, infinite beer and cigars, and making everybody in the universe throw him a birthday party), but also he has a connection to the Space Gem and thus can teleport wherever he wants. He was Adam Warlock and Gamora's sidekick for awhile, and has extensively been around Thanos.
Harry Styles is playing Eros, or Starfox by his adventurer name. To sum up a LOT of comic history in a short burst: There were originally only Eternals on Earth, but they had a schism and some of them left to go live on Titan (the moon of Saturn in the comics, not a different planet). My guess is that they'll alter this in the MCU to just say that Arishem had multiple teams of Eternals, not just the ones we saw. Anyway, the leader of the Titans had two children, Eros and Thanos. Eros went on to be an Avenger for some time, though he's been written out of recent comics since his power is to affect emotion, especially making people fall in love and have sex while under the influence of his ability. Honestly kinda wondering if they're gonna keep that or change it up somehow.
The post-credits scene is Dane Whitman about to touch the Ebony Blade. In the comics he's known as the Black Knight, and the sword is a relic from Arthurian times (they even snuck in a reference in the movie when Thena is wielding what turns out to be Excalibur). Also a longtime Avenger, the problem is that the Ebony Blade is cursed and amplifies all the most negative aspects of the wielder, which it feeds upon in exchange for giving the user all kinds of increased power. All of Dane's ancestors would have been previous Black Knights, and all were originally heroes who became murdering butchers after years with the sword. Seems like the voice was Blade asking him if he really wants to do that.