Could all the sequels make sense if...?
OK, admittedly I haven't seen all of the TV series or The Source and fans of the series swill probably want my head for this because it allows for Highlander II to exist (although some of them don't consider III- or even the original?!- to have happened either) but this is my theory of how all the sequels could actually make sense together.
As a general film fan (rather than a massive Highlander franchise fan) I consider the original a true great of Action / Fantasy cinema and do not relish the idea of an inevitably characterless remake. I'm not even going to attempt to make any sense out of the animations (although I've heard The Search for Vengeance is actually the best Highlander since the original). The second and third films are entertaining but extremely flawed at the most basic level: story.
Endgame I just found a messy follow- up to the TV series with it's low budget and pedestrian villain (although, frustratingly, there were some good moments and ideas in there) and I'd imagine The Source is just more of the same. Despite the titles, neither offer much in the way of an ending or any kind of resolution at all now that it looks like it's all over, so...
1985: The original film takes place. The Gathering begins. Vasilek, Fasil, Kastigir, the Kurgan and Connor think they're the last five immortals left in the world. Connor takes the Kurgan's head and the quickening’s so incredibly powerful he thinks he must have won the Game and become the One.
1991: Seven years later (just about) Highlander III takes place. Brenda didn't die in the car crash but has left Connor. Kane awakens and Connor realises he clearly isn't the One. He kills Kane then decides he is again. But Alex leaves him, taking their adopted son with her.
1992 - 1998: The series happens. There are obviously loads more immortals in the world (Connor got back with Brenda in '95 but she died of radiation).
1998- Endgame takes place. They're just rounding it off to the nearest decade when they say Connor's been gone ten years, it's actually just seven. Anyway, Duncan kills him to defeat Kell.
1999- The Source happens (it's set in a near future dystopia so that’s your late 90's of Highlander II).
1999- Sometime between The Source and the shield going up in Highlander II, someone- perhaps Duncan- does what Connor does with Ramirez in Highlander II and resurrects him.
1999 - 2024- At some point (probably quite early on to allow time for ageing) the Gathering ends and the resurrected Connor finally does actually win the Game.
2024- Highlander II happens, bringing the sequels full circle.
Or you could just forget all that and pretend only the original exists!