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As there were a few discussions on linearity of official quests and I claimed that non-linear foundry quests are possible, I come back again with proof!
Campaign:- Maiden in the Moon
Quest:- The Wild Moon(mote) Chase
... is a non-linear quest. The campaign will be non-linear too. Infact next part will be much more "sandbox" than previous quest. For now, this much should do.
Hence my argument that cryptic could have easily made "sandbox" or non-linear quests(even now they can) but due to storytelling purposes they kept the quests linear.
Nonlinear quests also bring with them
"Its too confusing", "I am stuck", "It is undoable", "Can not progress"
complaints as soon as players hit the Puzzle or Explorer part. This also shows that "sandbox" type quests cannot become popular among mainstream MMO players.