On the face of it, Top Secret is an odd place to keep digging for experimental bits. Back in the day it was—as most of the historically-themed second-generation games—an awkward cousin of the TSR family. Nowadays with little to no current following, we can safely and sadly say it's in the dustbin of gaming history.
Still Top Secret, in it's pre-TSR publication days as Spy World, had it's roots in the mid-70s creative explosion of experimentation with rpgs and as such you can still mine curious bits from it that may either shine a light on alternative paths not taken or be of use for classical play campaigns here and now. ( I pointed out a few months back on this blog TS had trace elements of early concepts that mostly dropped out of the fabric of rpgs: competition and GM-player role blur.)
Another interesting coulda, shoulda,woulda-been TS concept was the idea of a character-based sandbox campaign (as opposed to the mostly location-based sandboxes that we many of us know and love in classic-play D&D or Traveller). I say coulda-been because while the idea of a game based on the non-linear unraveling and exploration of a character-based espionage network was explicitly laid-out in the campaign section of the original rules, game-play and the published modules became quickly centered around the commando raid-like clearing of Bad Guy facilities.
Which is a shame because an interesting space between the old location sandboxes and the heavily-plotted nightmares that marked each successive edition of D&D was lost. Let me back up and break down that forgotten piece.
TS's campaign rules provided several ways to portray a network of NPCs. (For ease of reference I present the page in its entirety on the right here, click to embiggen). There were several layers of the network from the Administrators at the top (which in Spy World were originally playable characters) .
More interestingly there was the use of six different types of communications links inside the network complete with symbolic notations for each: a line with arrows pointing in both directions for a direct connection or a broken line pointing in one direction to signify a dead drop.
In theory, this would be used to make a symbolic map. Instead of hex or area location maps linked by roads, tracks, and rivers you had nodes based on the various agents linked by how they are interfacing with the rest of the network. Play presumably would revolve around eliminating, duping, tracking, spying seducing, etc. each personage along that network in an attempt to break or infiltrate it.
You saw the attempt to use this model in the introductory module, the infamous Operation: Sprechenhaltestelle. Its implementation was a muddle though; the scenario taking place in a fictional spy-ridden Cold War neutral town nestled between East and West typically ran as a locale-based town and dungeon crawl with an awkward attempt to graft the network rules on top.
It wasn't until the release of the Top Secret Companion with its sample campaign scenario Operation Meltdown did you get the full effect of Merle Rasmussen's, the game's creator, intentions. In that scenario, you had a timeline for a conspiracy by a globe-trotting network of baddies spanning the fence from ninjas to South American Nazis.
But the timeline wasn't a plot railroad as much as it was a timetable of events that would happen if the players didn't intervene through their romping around in a non-linear. The focus of play was the team of players jetting around the world, figuring out which person fit where and how to muck up the plans. They could fly anywhere with the help of their various spy agencies, adventuring was completely independent of both the box cars of an adventure path and the restrictions of locale-based sandboxes. Interesting stuff.
To give you a graphic representation here is the comm links diagram for the adventure:
The style of campaign play has an intriguing amount of potential for a fantasy campaign too.
Say you set a hook, the players uncover a vast conspiracy of forces in the Archsyndocracy of Outer Kutalika to awaken the cosmic horror of long, dead elder gods. They chose to follow this hook and you draw up a network like above for a network scattered around the locales of your campaign world (you could seed it with agents living or visiting dungeons or other traditional adventuring locales to pad out play).
Or take something smaller and more confined, but filled with NPC intrigue: the Golden Khandive's royal pleasure barge; a drafty, snowed-in fortress; the gizzards of a sleeping god; or what have you.
Food for thought in the sandbox.