First of all:
Welcome to the forums!
As for your question:
This thread might be in the wrong section since your question is both about old lore and newer stuff, so
@Crowsfoot or another mod should probably move it to somewhere more appropriate.
Ok, now on topic:
As I understood it (I am not an expert though) the Great Plan wasn't so much about getting control over the whole world, just to control key positions and have magical influence to make sure Chaos doesn't win.
Chaos has been in the world for many centuries (or millenia? Not 100% sure) and didn't destroy the world, so....it kinda worked for quite some time. But yeah, with the End Times it becomes apparent that in the end the Great Plan actually failed. Chaos won and the world was destroyed.
Behind the scenes....yeah to me it does feel that Lizardmen weren't as thoroughly explored in the fluff as they probably should have been. All the descriptions about them I read sound as if the Lizardmen and especially the Slann are so powerful that they can hardly be matched by anything except some High Elves magic and that they should be there on any front fighting Chaos alongside those humans and dwarves and elves and so on. But then it seems that most stories aren't centered around the Lizardmen at all but focussing on that Karl Franz dude and his pals?
Perhaps somebody with more clue about the fluff than me can help us figure out if and why that's the case. Perhaps humans and Elves just make better main charakters for stories....
Now about the newer (AoS) part of the lore:
Everything is centered on Sigmar and his dudes now. The Seraphon are often mentioned in the fluff and they are certainly characterised as both powerful and devoted to fighting Chaos, but as of now they haven't been doing all that much.
And again behind the scenes:
They just took all the old models and made warscrolls for them. It looks a bit uninspired because that's what it is. They did that for all armies.
And then as
@Crowsfoot said: For some armies they make new models and fluff for them, and some are basically "phased out". First they don't receive proper updates anymore (like Greenskinz), and then they might disappear completely (like Tomb Kings).
That depends on many factors, of which some are in-universe (fitting the story or aesthetics) and some are real-world based (productions and whether GW can make profit with them).