Looking at the first few seasons of BTVS.

[NotASlayer]

Kind of true but season 6s failure would explain that.

See I think the "plan" was in seas56 they break everything, in 7 they fix things, then in season 8 they would do something new.

No season 8 forces them to try to both fix everything and do something new in 7

However, they have admitted there was no plan for season 6. Probably because it was done with no prep thanks to being renewed (possibly after The Gift was filmed; I have read, non-writer, comments to that effect) and no ideas for episodes.

Now, if there had been planning, the First's arrival could have been foreshadowed in S6, with it being behind everything, like the Mayor was foreshadowed in S2 and Dawn in S4. I think that in truth that S6 was just the random mess of unconnected garbage it looks like.

 

[NoShip]

It has been said that Joss wrote the seasons of BTVS, that makes me consider what a useless scriptwriter he was.

Interestingly enough, in the late 1990s Whedon was considered a decent to pretty good scriptwriter. The Industry never considered him a great scriptwriter. However, and this is what made Whedon a "star", he was considered one of the top ten script-doctors on the planet. Opened a lot of doors for him. Got BTVS for us.

 

[ILLYRIAN]

Admittedly he did write some quality scripts before BTVS but having to plan what his characters were doing over the years of BTVS was often his downfall.