Yeah, I do feel like it's pretty likely that Luffy will get at least one more alternate form in this manga. This is a Shonen battle manga at the end of the day. And a last-minute powerup for our hero during the final battle is almost a requirement.
I HAVE imagined that there could be a "Mangaka" Luffy, if he goes FULL God-mode. With Luffy having to help draw the ending himself. But, that feels a little too meta for Oda, frankly.
Luffy could get some sort of "Demon King" form. Imu probably has his concerns about Nika for a reason. So, I could see a truly "Unleashed" form of the Nika Fruit looking kind of monstrous or demonic. But, that wouldn't really fit Luffy's personality.
Maybe it's simpler than that. Maybe Luffy's final form is literally "The Sun"! Becoming a replacement for the Forbidden Sun that was likely mined out of the center of the planet during the First World. But, that seems to imply a pretty final sacrifice for Luffy, and I'm not so sure Oda would truly kill Luffy off.
If Devil Fruits really are all about different potential futures people have imagined...maybe Luffy won't get just ONE new form...maybe he'll get ALL his potential futures at the same time? A entire "Luffy-verse". A Luffy that becomes a doctor. A Luffy that becomes a cyborg. A Luffy that becomes the Pirate King. A Luffy that becomes a homeless bum. Every future that could possibly exist. That...really feels more like a version of Bonney's final form. But, it does feel like something Luffy COULD do. And it would at least make for a fun splash page during a final battle. And Oda LOVES coming up with new designs. And this would let Oda go wild one last time re-designing his main character in every way he can possibly imagine.
So, I don't know. I am expecting...something. Some new form or power up. But, it's hard to say what that's going to look like.