One that we ran on brexitmodeller was an eduard weekend in a weekend group build that we ran over Xmas last year. Eduard donated prizes and it was a quick wham bam thing over the Xmas fortnight. If was a success because:
A, it was short
B, it had prizes
If I were to suggest one here I would perhaps look at an OOB build that could be done quite quickly before enthusiasm dies down. We tried a 100 years of RAF starting on april 1st finishing on 15th sept for obvious reasons that petered out quickly because it was just too long.
So thats my two pemneth.
By the way you can build an eduard weekend kit quite successfully in 48 hours