I'd like to be able to highlight portions of text in emails I receive from other people, in Gmail. When I reopen those emails (often large), I'd like to be able to see what portions of text I had previously highlighted.
Not within the Gmail web client. I would recommend creating a Google Doc, pasting the contents of the message into it and marking it up there.
You could then take the URL of the Doc and put it in a reply to that Gmail thread addressed only to you as a quick way of referencing it in Gmail.
If a group of people are collaborating on this content, using a Google Doc instead of an email thread may be a faster, more efficient way of getting the work done.