The Star Wars story is a saga not just a series of random episodes set in the same galaxy.

That would mean further tales would have grow out of what happened in the past.

Leia being Vader's daughter is a plot element introduced in Return Of The Jedi and very little came of it.

She is a politician Force sensitive the last one of those we saw in the saga founded the Empire.

I can see her desperate to hold onto what she fought so hard for tipping over into darkness.

She was tortured by her father, lost her homeworld while he forced her to watch, nearly lost Han so this monster could test a potentially fatal procedure he intended to use on her brother.

Then there is the sibling attraction thing.

The potential for a really screwed up villain is there.

Just having her standing in the background for five minutes would be ridiculous, it would be throwing all that character potential away.