You wouldn't put him so dramatically in the teaser trailer and bring Ian out on stage if it were just a hologram or one of the pre-recorded droids from Battlefront. It'll be more than that.
I tend to figure he did die in Jedi, he's physically gone, but his consciousness/influence has been active ever since. Somehow. Like we know darksiders don't "force ghost", but Palpatine's also made out to be something as more-than-the-average-Sith-bear in both old canon and new, I could see him being around as something else, something new. With Luke probably the lightside equivalent, both of them not quite "force ghosts" but something up-and-above that.
So Palps has been orchestrating Snoke and turning Ben, he's probably what we know from the books has been "watching over" Ben since he was a fetus from the Beyond that Leia senses. It wouldn't have to be as direct as actually controlling Snoke as an "avatar", possessing him, more just some sort of less tangible darkside influence, more ethereal. Snoke wouldn't even need to know it's happening. I really doubt it was as direct as Palps speaking through Snoke (their personalities are similar, but they're different too), Snoke'll be his own separate entity, but more like an unwitting pawn. He's been carrying out Palp's plans all along, without knowing it, that type of thing. Darkside mojo.
That way a) Rey & Ben dynamic is still the main show, the meat of the story, with Ben as the physical threat and with his own agency/agenda, ability to eventually break out from under Palpatine's sway, B) you don't have to devote huge screentime to Palps in order for him to still cast a huge shadow over the whole movie, RotJ screentime would suffice, and C) you're more coloring in the blanks of the trilogy so far in a way that makes sense, than introducing totally new concepts that don't fit.
Get the sense a lot of that opera monologue in III is going to play into all of this. Like Sidious actually did learn his master's secrets before snuffing him, he's not quite a force ghost but he's not actually surviving either, it's more just his "essence" or whatever you want to call it. There's precedent for it in canon with all the Momin stuff, and the animated shows have opened up that more spiritual side a good deal more.