Of all the Marvel film franchises, no-one could have predicted that Guardians of the Galaxy would have such a meteoric rise to fame. With Vol. 2 currently smashing it at the Box Office and Vol. 3 already confirmed to be on the way, we're likely all going to be hooked on a feeling for quite some time to come.
In fact, the Guardians have been such a success that they seem to be moving to the very forefront of the MCU. James Gunn has stated in interviews that Vol. 3 will serve as both a conclusion to this iteration of the team and as a starting point for the next decades worth of Marvel films.
With so many plot threads being set up and the Guardians themselves undergoing a big change in their lineup, Vol. 3 is pretty much guaranteed to introduce a lot of new characters. We already know that Adam Warlock will be making an appearance, but Marvel still has a huge treasure trove of characters that could be brought into the MCU, either as new members of the Guardians of the Galaxy or as threats for them to face.
Some of these characters are a bit out of left field, but others have already had the groundwork laid down for them. Either way, any of them would be a great addition to Guardians of the Galaxy Vol. 3.