This isn't her first sequel
As album titles go, yes, of course this is her first true "sequel" but Madonna has been referencing, and invoking her previous works throughout most of the second half of her career. And as we are seeing with the sound and vibe of Confessions II, she isn't one to repeat herself sonically. 05's COAD was very disco pop house centered whereas Part II appears to be influenced more by detroit techno and even early 80's NYC post-disco.
Here's some other examples where I feel Madonna has made something almost equivalent to "sequel" albums already:
Like a Prayer — Ray of Light: a spiritual (pun intended) successor to 89's LAP, ROL covers very similar lyrical and emotional territory (family, loss, religion, spirituality, relationships), all while using "warmer" sounds, Like a Prayer was full of live drums and guitars, something ROL also utilizes.
American Life — Madame X: Madame X covers themes very closely aligned with that of American Life, just through a slightly different lens. Politics, social media, violence, are covered on both albums as well as overt overtures of love and yearning.
Erotica — Bedtime Stories: In my opinion, BS has always felt like a sequel to Erotica. The sounds may be updated and different, and "sex" has been replaced with intimacy and romanticism, but at it's core it's Madonna continuing to explore her sexuality and womanhood and there's an overall tone of isolation and the need to be seen and feel connection.
Madonna (debut) — Hard Candy: this one might be a bit of a stretch, but I feel like Hard Candy in some repsects was meant to evoke and hark back to her debut album. There's a rudimentary (complimentary) quality to a lot of the lyrics and production that reminds me of tracks like Physical Attraction and Lucky Star. It's a return to dance and moving in a way we hadn't really seen since those early years.
Just some thoughts considering some fans and casual listeners don't quite understand why she's making a "sequel" album - but in my opinion she already has a couple times over (to great or lesser extents obviously).