The missions that are available in the beta client have to be done in a strictly sequential fashion, even though most of them are not in any narrative sequence. This is more than a bit annoying in that if I get utterly stuck on one step of one mission, I can't do anything else until I beat it. I think it'd be wise if mission groups could be started independently. You'd still need to complete any missions that form a larger narrative in the correct order, of course.
I, of course, agree that otherwise unrelated missions should be available irrespective of each other. I do, however, believe that it will indeed be possible, and that the current restrictions - given the age of the missions - is more an indication of the system being tested back when the missions were released, than how things will "actually" progress. So, for example, you should (hopefully) be able to start on either "Anashel Sandbox" or "Jarobi Sandbox", irrespective of your progress in one or the other, but you won't be able to access "Jarobi Sandbox 2", before you're completed the first mission in that line. There is, of course, always the caveat that two sequentially linked missions might not appear as related in terms of narrative. Not before a later, 3rd mission ties them together - the only linking factor between the two apparently unrelated missions until then, being chronological. That being said, there are also merits to "bottle-necking" Agents on a certain mission, giving them few alternatives, namely in the sense that it will - albeit somewhat forcefully - incentivize Agents to seek out co-operation in the face of a particularly hard mission, rather than continue the "lonewolf" path and simply jump on to another. Of course, everyone might need a break from a current mission, so both aspects have an advantage to them, for which reason I think it should be somewhat carefully balanced. To take an example; a system where Agents are neither predominantly "spoiled for choice" nor "hard-locked to a single mission" - by, for example, having 2 concurrent narratives going, so people can shift focus from one to the other, but still be likely to get horribly stuck on both, if they don't opt to try and brainstorm/co-operate with other Agents.
