I was thinking (I know, dangerous), People seem to really like the Mission Chains in the DOFF system. It may be an oversimplification but basically completing a step in a chain mission opens up X number of other missions (one of which is typically the next step). So, what is to prevent one (or more) of those additional missions from being step 1 of another, related, chain?
Basically, what you are describing is a branching chain. These are currently tricky to implement, and the UI doesn't support it well. Borticus' Fugitive chain was originally structured this way, and wound up being restructured slightly as we ran into some issues.
Basically, what you are describing is a branching chain.
While a branching chain would be great eventually, and is the natural final product of my suggestion, my suggestion was more having a separate chain starting from within another chain. For instance, having say step 3 of 7 be required to be able to start a different chain (much like level 40 is a requirement for Children's Toys)
To use existing chains as an example...
Consular Authority: Karemma
Completing Step 2 (Secure Consulate on Karemma) opens up as one of it's additional missions Step one of the Karemma Colonial Team (Initiate Contact with Karemma Colonization Team). Both chains are still their own chain (UI doesn't change I wouldn't think) you just can't start the Colonial Team Chain until you Secure the consulate. The Colonization Team Chain isn't technically part of the Consular Authority Chain, just a pre-req for it. You would just set the same type tag on the start of the Karemma Colonial Team Chain that you would on say the repeatable for picking up the Strange Artifacts.
Now, obviously I don't know all the coding involved, just wanted to clarify my original suggestion.
If you can enter 1 follow-up mission for a mission, then it is not possible. (You could make 10 mission that all lead to the same follow-uo mission, which is useless)
But if you can enter 1 prerequisite mission, then you could create 10 missions that all require the same 'start-mission'. (But then the GUI thinks the start mission is part of all 10-chains)
Children's Toys, for example has at least 2 missions that the first mission opens up. It opens step 2 in the chain as well as the repeatable for step 1. In addition, the repeatable for step one doesn't make the GUI think it is part of the chain. So I'm not sure where the problem is...