The flip-side to the trading thing for MP games...
Let's look at the example of the Casual Gamer VS the Hardcore Gamer, or if you prefer, the Utterly Screwed Player VS the Been-Winning-The-Whole-Game-From-Turn-Freakin'-One Player.
Let's say Casual Gamer has the second or third piece of a Schematic, (That's what I think they should be called, Spell or Item, it makes decent sense for both, though less for Spells,) that Hardcore Player needs... Now, Hardcore Player, being who they are, is kicking everyone's butt... But Casual Gamer, who's about to get steamrolled, suddenly has a bargaining chip, making an alliance or a mutual victory for them and Hardcore Gamer a possibility. (Though not a certainty.)
Hardcore Gamer, being who they are, is of course going to take the next part of their schematic so they can BBQPWNSAUCE their next single player game and watch they Volcanic Golem of Hellish Destruction wreak havoc on their opponents. I'm sure they also want the achievement, but personally I'd have a hard time getting excited about those in a game like Elemental. (That's just my opinion.)
Now, it's not just a collectible thing, it's a valuable resource of sorts, useful -only- to actual players. If you don't play MP, then it shouldn't matter too much, since you can still always play SP and just try to get them that way.
We've already established that, for this to work, MP and SP games both have to count. AI's need access to it at this stage, but the AI is crummy right now... After the brunt of the AI gets implemented, we'll see how much of a game breaker it would potentially be, but even then, getting AIPwned every so often never hurt anyone... Well, okay, that's actually not entirely true... But it only ever hurt a very rare few, so the risk is negligible. =P
The frequency between games in which you find these Schematic Items would need to be decreased a bit methinks, but it seems to me you already know that.
You could also have it so the Schematics themselves are much more easily found and all collectible at any given time, but make the items required for these schematics the truly rare things, and only allow a finite number of those items to be carried. Somewhere between Three and Five items seems good. Three seems best in my opinion. That allows you to increase the frequency between finds enough to make it manageable, as well as not too easy to find. Have the Schematics themselves work similar to Achievements, so you can keep track of all your formulae, including the one you're working on.
Now one of these super-weapons every 500-ish games seems a little steep... Problem is, unless there are lots of these super-weapon type items/spells to get, that's actually not too far-fetched, which might be a turn off, but at the same time, most people would probably ignore after their 20th game without getting all the stuff they needed, so hopefully, the loss of that won't impact them too much beyond time wasted and effort spent. If you could put lots of different super-items/spells in, then you could decrease the frequency between Key-Items found, making it a fairly tradeable resource for MP. It would rely on luck whether or not you were playing with someone who was looking for one of the specific items you possess, which may not necessarily be a bad thing. Having a Super-Bargaining-Chip every game is also potentially a game-breaker.
So, so far:
This needs to be a Single Player, Skirimish only feature. No using these in Campaign Mode or other Scenario's.
MP and SP games must count.
Schematics could be infinitely collectible, while the Key Items for completing the Schematics could be much more finite.
A high number of Schematics is necessary, in order to increase the frequency of finds in general, without directly increasing the frequency of the specific Key Item you might need. (Notice I didn't say 'Could' this time. This really would have to be a must.)
Each Schematic, once finished, should come with more than just an achievement or the like. A (relatively) detailed lore entry regarding the Weapon/Spell/Thing would also be unlocked, adding more reason to finish the schematics, and more flavor than just being able to essentially insta-own your next single player game. In my mind, this lore entry should come with a cool 3D model of the Item/Weapon/Creature you unlocked. I especially like the idea of putting together a Diamond Golem or a Glass Dragon, Artificial or Artificially Controlled Creatures in general. (I.E., you find the Golden Heart Scale, Dragon-Speakers Tongue, and Wand of Fusing to incant the words to bring the all-feared Great Gold Dragon under your command for a game.)
Given the above paragraph, Items and Spells aren't the only limits to this idea. You could even go so far as to get Schematics for special buildings that proffer special bonuses or allow the creation of a special unit for the duration of a game.
In later versions, if the AI having access to these items ends up being too big a deal, you could easily re-implement it as a quest for only the player, force them to accomplish some super-dangerous task before finishing the game in order to obtain X Key Item.
The Key Items themselves should have a lot of flavor just in their appearance. Lore entries for all of those would be too much however, especially at three per schematic, and I'd guess you'd need somewhere between 50 or more schematics to make the frequency of finds high enough to help the player to not lose hope in ever finding the ones they need.
You shouldn't have to play more than maybe 2 weeks worth of ANY game-length/size to complete 1-3 schematics. This will allow players to pace themselves, and give the game 3-6 months worth of replayability just to complete all the schematics. This is also considering a relatively casual pace, maybe slightly more than what would generally be considered casual gaming.
This time can be shortened drastically via MP games, which is a good thing, bringing the whole thing down to 1-3 months of semi-hardcore playing and some smart trading with other players.
Trading between players doesn't necessarily have to be in-game either, it could potentially be done in the lobby for those who are just looking for the achievements/lore entries. This is especially good if someone has some time to kill, but not enough to get started playing a game.
I'll pitch in more later once I think of something else to add...