Do you mean a way to make it so you HAVE to play one particular 5-mission arc before you can play its sequel 5-mission arc? No, there's nothing like that. If you meant a particular clue letting you branch into a different mission, we don't have branching yet either.
I recall LittleBigPlanet (another user-level-design focused game) had a way to put keys to other levels in your levels, but that's the only example I can think of.
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Basically, yes.
My original plan was to have something like a 2-3 mission arc. On the last mission, there would be a "hidden" Object (can you physically place objects where YOU want?). If the player/players actually found said object, then they would have access to a "locked/hidden" 5-mission arc. That's the only way to access this "hidden" arc. From the sounds of it though...I won't be able to do this. *sigh*
No can do, yet... maybe in the future but, with the way this game plays, I doubt it.
I believe that a Kheldian Gold Standard should be based on SO's, and for anything above that... there's Platinum!
Save Ms. Liberty (#5349) � Augmenting Peacebringers � The Umbra Illuminati
Save Ms. Liberty (#5349) � Augmenting Peacebringers � The Umbra Illuminati
Maybe have the final "clue" lead the player to the next arc...
or you could even have this allow you to branch a story arc
example:
"Well you have recovered one half of the talisman of Doooooom, the defeated Silver Mantis mentioned as she was being led out by Longbow that the second half was located in an Arachnos base on Port Oaks......obviously a trap! Do you;
a) Walk into the trap, you never know, you might get lucky.... Arc #2846
b) Take this fight to the source, hunt down Black Scorpion in Grandville and take him by surprise!....Arc #2946
Kinda like those "choose your own adventure" books I used to read as a kid (maybe I just dated myself hehe)
Oh no someone close to my age 
The development team and this community deserved better than this from NC Soft. Best wishes on your search.
So I haven't fully messed with the system yet, but I'm wondering if there's a way to "get item" in one mission that will "Unlock" a new mission on a different Arc. I looked at the "Collect an Object" settings and couldn't find anything in there. I also tried looking at the "Clues" section of the creator.
I'm pretty sure it doesn't exist, but I'm interested to know if anyone might have found a way to perform this.