I always saw the Agatha/Bertha/Karen theory, but wasn't sure on Glacia. Though the more I think about it, I can kinda see your point. Glacia is said to be a foreigner, and her typing makes her suitable to be from Sinnoh, where Bertha is from.I've always had this stupid headcanon where I think Agatha, Karen, Glacia and Bertha are all related somehow. Like Glacia and Karen are cousins and Bertha and Agatha are their respective grandmothers or whatever. It's a silly headcanon...but it's one that I can't shake since forever.
Speaking of Karen, I've always thought she was an ex-Beauty due to her original sprite looking similar to to the R/B/Y Beauty sprite. Even her updated look favors the D/P/Pt Beauty a bit.
I always found it odd that we never received an explanation for Agatha's disappearance in Gen 2. Lorelei got one in Gen 3, but not Agatha. My headcanon is that she retired, but gave recommendation for her grandaughter (Karen) to be an E4 member.
Now this one headcanon I had is very left field, but I felt like sharing. The theory involves Lorelei and Misty. In the anime ( I know anime evidence is sketchy), Misty's sisters are all named after flowers in both the English and Japanese versions. Even though Misty's name does not follow this pattern, one meaning for her Japanese name is a type of flowering plant. Lorelei's Japanese name can also be translated as a type of lily. It is said that Misty has aspirations to be similar to Lorelei (probably inspired by the anime). My farfetched theory is that Lorelei is a relative of Misty.