I would actually love to see STS-51L and STS-107 simulated in SSM someday as well. As mentioned by mborgia, especially STS-51L would be an interesting mission.
I think it would be a nice tribute to have these two missions simulated in the way how they were intended and planned, to give the user the opportunity to complete them.
But I also think that, for now, there are many other very interesting missions that could (and should) be simulated first.