Ace's unknown fate at least means you have room to make something up if you find it convenient. Mel wound up leaving to keep an eye on Sabalon Glitz and lord knows if she ever made it back to Earth, though I gather some Big Finish or books claimed that Peri and her husband eventually did so. Also, there was Tegan walking out on the Doctor on Earth in the 1980s, though her opinion of him might not be such a positive one, unless she'd been able to look on the bright side after a few decades.