When I talked about scriptless plotting, I approached it from the traditional top-down perspective: you have a villain at the top, you figure out what the villain is doing and who the villain's major underlings are, then figure out what the underlings are doing and who their underlings are, and so on down. This "figuring out" can be either all done in advance (the traditional way) or by random rolls, like the 1 in 6 technique I suggested for connecting random elements to the villain.
But it works the other way, too. Instead of connecting a new NPC or low-level monster to an existing villain, leave the villain slot open and give a flat 1 in 6 chance that they're working for/connected to someone a little more powerful. Roll for a wandering monster or urban encounter (your preference) to find out what, think up a good reason why they are connected (or improvise one, based on one of those infamous 3d6 left-to-right rolls.) Improvise a couple clues to this connection, if desired. If the party investigates, there's a 1 in 6 chance the new guy is connected to an even tougher NPC or creature, and so on... the plot evolves through play.
Here's an alternative technique for a more web-like set of connections between NPCs in an urban setting (although adaptable to other settings.) Start with a d6 table with six empty entries. When the party meets an NPC (randomly or by seeking them out,) roll 2d6; if the dice match and the number rolled is still an empty entry on the chart, add the current NPC as that entry number on the chart; that NPC is now considered temporarily significant. In the future, when you roll 2d6 for a new NPC, if the dice match, the number rolled indicates a connection to that NPC number.
Example: the party seeks an armorer and you invent Hagar of the Hauberks. When they meet him, you roll 2d6 and get double 3s, adding Hagar to line 3 of your signicant NPCs chart: he's up to something, or has a lot of connections, or has an old enemy... you can leave his significance undefined. When they later meet Rupert the Robber, if you roll double 3s, Rupert is connected to Hagar in some way: perhaps he's planning to rob Hagar, or is Hagar's black sheep brother. Roll a reaction roll if you're not sure how they're connected.
If the party decides to become embroiled in Hagar's life in some way, he stays significant. If they ignore him, then beginning next week, if the 2d6 significance roll pops up double 3s, Hagar gets replaced.
With this technique, PCs who are very active socially will help you fill out your social network with very little prep, but only the NPCs who draw the interest of the players will remain significant.
Edit: I intended this to be published tomorrow, but screwed up the date/time, so everyone gets an extra post for today.