Scaling isn't a perfect solution, I would readily admit. It just happens to be the best option if you can't afford to dedicate folks to tailor made encounters. If I were to do it, I would create an endgame quality mission as a kickoff mission for each season and make it replayable with skill modifiers a la nightfalls. This mission would also feature the juicy lore tidbits to kick the narrative off. From there you could introduce the new group activities/zones/whatever much as they do now. Ideally these missions would also have some cool loot worth chasing to encourage repeat play and make it worthwhile for everyone to devote time into it. You could also set it up so that a full fireteam could go in, and have the enemies scale in difficulty so people who prefer to group up don't feel like they're being forcefully separated, as its never fun being told to play in a way you don't enjoy. I guess my tl;dr would be that while sure the mechanics will always be limited, there are still a lot of ways that interesting and challenging solo encounters could be setup.