George HW Bush. Jimmy Carter. LBJ. Hoover, if you don't mind going back nearly a century. And before you say "well that's just three examples!" there have only been 45 presidents in 250 years. Just about the only presidential thing you can get dozens of examples of is white men.
I'm not sure how "Well Trump got elected" is any proof of your "parties alternate 2-term presidents" thesis. I doubt Sanders will run in 2020, but I don't pretend to know what will happen in the future.
You should add Nixon to that list, since they said two full terms, and Nixon's example may well be followed again. Gerald Ford belongs on that list too. In fact, only four of the ten most recent former Presidents served two full terms. (JFK is something of a different category, of course.)
Carter and Bush both lost because the economy was in a recession during their final years. Bush might have won if the economy had pulled out quicker. LBJ did not run for reelection.
If the economy is doing well, it is quite possible for Trump to be reelected.
I'm not sure how "Well Trump got elected" is any proof of your "parties alternate 2-term presidents" thesis. I doubt Sanders will run in 2020, but I don't pretend to know what will happen in the future.