Out of any character out there, Adam Warlock is the one that makes the most sense to be introduced in Phase 4. Also known as just Adam, the character was introduced in the 60s, appearing in comics from the Fantastic Four and Thor series.
Since then, Adam has been an integral character in many Marvel plotlines.
Adam has actually already been teased in the MCU, in one of the five post-credits scenes in Guardian of the Galaxy Vol. 2. The one in questions shows the leader of the Sovereigns, totally defeated and staring at a new birthing pod which she says is “more capable of destroying the Guardian of the Galaxy.”
She then calls it Adam, just before the scene cuts.
While Adam Warlock has generally appeared as a hero, there is an incarnation of Adam called The Magus that serves as a villain, representing the dark side of Adam.
In terms of powers, Adam Warlock might be one of the only characters who could go toe-to-toe with Captain Marvel, possessing many of the same powers and a control of cosmic energy. Considering Phase 4 will probably be leaning into the whole “Cosmic” thing, Adam Warlock is inevitably on the way.