A bunch of cocooned desiccated corpses including a fiendish black dragon which he may possibly think to loot for parts and also something huge and hollowed out. And a few live critters. Then the strand starts shivering... run, hide inside the huge monster, cut free the live critters and fight or (gulp) talk?
Two qlippoth floating down along the strand with some idea of messing the place up. They're hostile to just about anything, but they might be willing to talk initially if challenged strongly enough. Among the things they're hostile to are each other. A tricky wizard with illusions or a good line in patter might be able to get them to break their truce and fight each other. Outrunning them until a branch in the strand is certainly possible with a nightmare mount too.
An ettercap and some minor spiders slowly spinning a new branch onto the strand. Beatable in a fight.
So all the other PCs are dead? Maybe the new player characters then?
Neutral humanoid NPC tending to a cabin w/ protection from demons. Some sort of archeologist/dioblist but for demons (idk what they're called) who rarely leaves the cabin. Protective magic is weaker when two people in cabin, wants PC to leave. Higher level than PC but does not want to kill them. Could be convinced to let PC stay if PC finds new information/artifact from somewhat dangerous location that PC has proper spells to bust into.
ANOTHER NIGHTMARE (or pack) WHO DOESN'T LIKE THOSE WHO BETRAY THEIR KIND.
Battle between random demons, PC can pick a side and turn tide.
Idk if any of these are plausible but I might use the NPC w/ cabin as a plot point in my own campaign :) (already a theme of cabins lol)
Bebeliths are always fun and as a bonus spider themed, though not a good match for a 7th level PC (they're CR10 I think).
A succubus. Up for conversation (interested, curious, wants OUT OF THIS FREAKING MESS OF SPIDERS) and is willing to help the PC if he can get her to a nearby gate to the Prime Material Plane and through it.
The gate, or course, is guarded by a glabrezu... can they sneak past him, or deal with him? Lolth hasn't done much with this gate yet, just sent out a few scouting parties, but they haven't reported back yet. If negotiated with, he might let them through (for what payment?) or even grant a twisted wish (but for what payment?). It might want one of them to give it the other, or maybe it wants them to find out what's going on with the scouts... but does the gate end up on the PC's own Prime, or somewhere else that Lolth is planning to conquer? This might be a good way to meet the rest of the PCs if you want to introduce the new characters at this point (they're in the world on the other side of the gate, or if they're evil maybe they're part of Lolth's initial scouting team on this plane.) Rewards could be great, but the danger is equally high (and the succubus could be a nice recurring frenemy).
Depending on how graphic you want to get, some kind of demonic stallion that wants to mate with the nightmare. There might be consequences down the line... especially if the stallion is really a shapechanged demon of some sort (squick factor very high here)
A drider (CR7) or maybe a pair of them, who are just trying to survive the web, and are cautious (they won't attack without a chance to size up the PC first). They might be convinced to team up with the drow for survival's sake, but might also be just looking for a chance to backstab... and if he doesn't deal with them somehow, they know something nearby that might want to track the PC down and leave its gear strewn about the web strands (a retriever? a pack of giant spiders?).
Atmospheric: a pack of dretches, reskinned to all have the lower bodies of spiders, scuttling down the strand, singing in eerie unison... something about an itsy-bitsy spider and a rainspout.
Atmospheric: another strand of the web, some distance away (maybe just close enough to jump to), with a long procession of slaves in chains being escorted somewhere by skeletal drow.
Atmospheric: an itsy-bitsy spider crawling along the webstrand. It will scuttle away from the PC at first but try to follow him, maybe acting like it wants to be a friend.
Atmospheric: a swarm of demons around an angel getting increasingly wrapped in spiderweb. Probably not a good idea to get close to either of them, but a really daring PC might be able to pull something off.