Honestly, at this point, they should really just save whatever for a sequel. It's been two years. As a reminder (though there are extenuating circumstances here), Dark Souls 2 was 2014, and Dark Souls 3 was 2016.
I guess the new reality is that it takes six years to make a game and three years between expansions. I think I'd just prefer smaller games and smaller dev cycles.
Armored Core is one of their oldest running IPs and a series I have been playing since 2nd installment. AC6 was without a doubt the best one. Highly recommend.
Why? With a DLC they can expand on weaker areas and introduce interesting advanced mechanics, whereas a sequel needs a lot more groundwork and can't expand on existing story threads as easily without some repetition.
It's such a good game that I'd prefer more of it to a sequel, at least right now. Make the sequel it's own thing that's not burdened by having to finish all the unfinished stories.