The idea of a false-prophet leading people to their doom was done already on TOS, one of the worst (but amusing) episodes, The Way to Eden.
The fact is that it would have been very difficult to find a way to extend the character arcs of the TOS crew after Trek IV. They eventually found a vehicle with Trek VI, but it was not easy.
When you move from TV to film, it really does demand more core character arc movement. The reason the last X-Film movie flopped, for instance, was that it didn't really tie into the mythology and was really just an extended episode.
All the prior Trek films had higher personal stakes. Trek IV was kind of an extended epilogue of Trek III, with the crew just happening to need to save the world which then grants them a lenient "sentence" for stealing the Enterprise.
It's only when you get to Trek V where it seems like there really isn't much story left to tell. If they had decided to finally do a TOS series (ala Phase II) that would have been a good time to do it.
Introducing a long lost brother felt much too contrived. It was bad enough to insert an old flame and an illegitimate kid in Wrath of Khan, but to continue adding relatives we've never heard of before just strained credulity.
Really, once you get a certain point in a franchise you need to know when the well has gone dry. So it's not even a problem of the writers as much as the business decision to move ahead in the first place. The odds of coming up with a script that could extend these characters for one more film were stacked against them.