I've been in this boat for about as long as I've been aware that D&D had a class called the Fighter. Warrior's a
far better name for the concept.
With how many books have been written about the guy, I would've figured Driz'zt would fall under the Jodah Rule by now.
(Basically, if you've been around long enough, and you've lived through enough crazy adventures and emotional extremes, and your planeswalker spark hasn't ignited...you probably just don't have one. In other words, "if it was going to happen, it would've happened by now." Granted, Jodah was over 2,000 when he made that observation, but it was only his second or third book: 30+ books over 200-ish years probably contain a comparable amount of experiences that would ignite one's spark.)
Then again, if the decision was made to make him a planeswalker for the set, it
would make sense to establish it in his books, like how Vi was pushed as a character in Eberron, and Boundless seems like it's on the right timeline for them to have made that decision, then gone to Salvatore and told him to put it in his next book, so...probably shouldn't rule it out entirely.
Despite the above speculation about Driz'zt, I think this is more likely: when they give a character a planeswalker card for the first time in Magic, they almost always want to have the ability to use that character again in later stories and settings. Major existing characters from other franchises don't have that kind of freedom of reuse - because of the amount of ongoing dialogue between the different story teams necessary to keep continuity straight, if nothing else.
I think we'll get five new (or basically new, in the case of Vi, and maybe another character gets seeded in one of the adventures between now and next summer) characters as planeswalkers in the set, each being from a different one of the most popular D&D settings: unless the numbers have changed significantly since
these survey results in 2015, that's the Forgotten Realms themselves, Eberron, Ravenloft, Dark Sun, and Planescape.
If the Forgotten Realms set is successful enough for Magic to come back to the D&D well, the most likely places they'd go, barring just doing the Realms again (focusing on different areas than they did the first time: as
Dominaria showed, one Magic set can touch on a lot of different locations on a large, well-developed plane, but it can only
focus on so much), would be the settings likely to be popular enough to get similar levels of broad appeal. Establishing planeswalkers from those settings in the first D&D set would then give you existing "face" characters to follow into their personal stories on their home planes, and it would offer the potential to seed further interest in the settings themselves
through those characters.
So you've got an artificer from Eberron, some sort of psionics user from Athas, and probably a planetouched of some sort as the easy fits. I don't know Ravenloft super well, but I'm sure there are horror tropes it hits which Innistrad doesn't, and at least one of those tropes should prove a fitting source to build a planeswalker around. And...I have no idea what an "exemplar" of the Forgotten Realms would look like. If it was me developing these 'walkers, the FR one would essentially be a "fill" slot - figure out which colors still needs a PW after filling out the rest of the roster, then pick an interesting area of the Realms that would have a natural association with that color and start from there.