Raiding is becoming less and less optional

[Iscah]

I don't any of the other raids being connected except for Coils and maybe Alexander but I highly doubt those will ever be mandatory. I suppose we'll see though.

Other raids are also relevant:

Omega is the third piece in the puzzle alongside CT and Alexander - the third raid heavily involving the Ironworks, referenced in the Twinning (music and datalog 4.5) and also Tales From the Shadows, which reveals/confirms that Omega itself didn't "die" but transferred its consciousness into the OMG minion accompanying Alpha. Omega's control of the interdimensional rift is one of the key pieces of information that Future Ironworks needs to build their time-and-space machine.

Also, the Warring Triad. It's still to become actually significant, but has a number of elements that seem like they should be important: a survivor of the Thirteenth, continuation of Regula's story out of MSQ, another hint of what Elidibus might be working with... also dear goodness we need something to continue that plot line so we can get Unukalhai doing something more productive than hanging out in the back room twiddling his thumbs for the rest of the story. He's far too interesting for that - at a plot potential level, if not as an individual. Also the role quest finale in Shadowbringers seems to tie into some relevant information.