The Numbers don't lie. It has been 15 years since the release of TDM 1.0 in 2009.
I think we should consider a contest to celebrate this.
Further, the contest goal should be with the intention of motivating the completion of a campaign that can be deemed official.
Here are some possible ways forward:
Proposal 1: Middle mission(s)
Authors are asked to create a mission that expands the story between "A New Job" and "Tears of St Lucia". This can involve escaping the heavily guarded city perimeter, stowing away on a ship or caravan, attempting to recruit a partner for St Lucia and failing, getting caught by Builders before arriving and having to escape their compound, getting lost in haunted woods on the way there, etc. Any interesting way that the connection between the two official missions can be expanded. Points are awarded for making explicit references to Corbin, St Lucia, and any story elements in the two official missions. If the resultant mission is of high enough quality, it can possibly be made into an official mission. If we get a number of excellent submissions and they do not cause logical inconsistencies, we might even be able to add two or more to the official list.
Proposal 2: Intellectual Property distinct approximation of TDS missions
Over the years, many have asked that we recreate Thief 1 \ 2 in this engine. Obviously we cannot do this due to copyright law but we could create similar missions and stories that approximate the Thief 1 or 2 designs. That said, I think that most of our audience has played these missions to death so it may be underwhelming to see them arrive in approximate form anyway. What might be better would be to develop a similar story to TDS and make missions that resemble what T1 \ T2 players were dreaming would arrive when TDS was announced and the first screen-shots were shown in gaming magazines. So take any TDS mission you like, examine the story arc and wildly re-interpret something similar but on a much grander scale. Since it's possible that two or more contestants will choose the same mission to re-interpret, we have a slot system were authors need to claim their preferred mission and if it is taken then they must select another one or lose some story points. The slots represent a sketch of what the mission author might try to do rather than a blueprint. If authors can come up with a mission that has almost no resemblance to any TDS mission but would make for a compelling story development in a similar story arc then that can be claimed as an alternative to a slot.
Proposal 3: Same as proposal 2 but we stick with Thief 1 rather than TDS. Wildly re-interpret T1 based on what you may have dreamed of when reading the gaming magazines or playing the demo. Slots system to prevent duplicate submissions.
Maybe with either proposals 3 and 4, we still require the use of Corbin as the protagonist and a connective enough story that they could also be adapted as middle missions for the official campaign if the team agrees on it.
Proposal 4: Another "Connections" Contest. We just allow authors to connect any two missions with each other or expand the story of an existing series or single mission.
So those are my thoughts. I'll leave it to players and mission authors to suggest other proposals and if we have some sort of consensus about the most popular proposals then we will make a poll. Realistically between now and October we may not be able to hold a contest with any strict guidelines ( and tricky issues with maintaining a distance to Thief IP ) so proposal 4 may still be the easiest option.