I've always wondered if HPL had lived if he would have taken his own work in that direction.
I doubt it. Howard's pulp-action stories were a reflection of the man himself; he was almost incapable of writing any story that didn't finish with our rugged protagonist laying down a little two-fisted justice on the men or monsters that menaced him. He was a "red-blooded" kind of guy, who loved manly struggle and high-octane action.
Lovecraft was a stranger in that realm. The closest he ever comes to an action scene is the chase in "The Shadow Over Innsmouth," and the closest he ever comes to "pulpy fantasy-realms stuff" is the Dream-Quest
, which, besides being more "flowery" and poetic than your average pulp-fantasy, also marked the end of his experiments in that direction. I don't think he could write a Howardian fantasy story if he tried.
For an indication of the direction Lovecraft's work would have taken, let's look at his early material, starting with "The Tomb," "Dagon," and "Polaris," and his ending material, including "The Shadow Over Innsmouth," "The Shadow Out of Time," and "The Haunter of the Dark." The biggest difference between, for example, "Polaris" and "The Shadow Out of Time" isn't theme or aesthetics, but in the scale
of the stories and the tendency toward a science-fictional or materialist paradigm over a supernatural one (not to mention, of course, the quality of the writing). The same with "Dagon" and "Innsmouth," the same with "The Tomb" and "The Haunter." If he'd lived longer and followed the same general trend, I think if anything he would have gotten further away
from Howard and, perhaps, closer to Clark Ashton Smith, who laid on the bizarre cosmicism even thicker than Lovecraft did (to his detriment, it must be said).
Another possible glimpse of what might have been, in another direction, is the R.H. Barlow collaboration "The Night Ocean," the real last thing HPL ever wrote, even later than "Haunter." Compared to anything else attributed to Lovecraft, "The Night Ocean" is almost too
restrained and subtle. It deals, more than anything else in the world of Lovecraft, in atmosphere, suggestion, and mood. How much of that is Barlow and how much Lovecraft, I don't know, but after perusing some of the other Barlow entries, I'm inclined to attribute the maturity and quietness
of "The Night Ocean" to Lovecraft. That's the possibility that really excites me (and then saddens me when I realize we'll never see it developed any further). Lovecraft, throwing aside verbosity and italicized revelations to really settle down to the business of suggesting
the weird or supernatural and gradually building a mood
of gloom and menace. He did it before, in stories like "The Colour Out of Space," and it's worth pointing out that "The Colour" was his favorite among his own stories. I think "The Colour" best represents what he was aiming for all along, and as his skills grew, he got better and better at pulling it off. So maybe a surviving Lovecraft would have gotten more quiet and suggestive, eschewing huge tentacled monsters in favor of tiny hints and manifestations of the Unknown.
We'll never know. In either case, I think he would have gotten less "pulpy," not more.