In regards to Slaanesh, they've confirmed more models are coming this year, and while Slaanesh's freedom will surely be a huge story arch, it doesn't necessarily need to be linked to the battletomb. Slaanesh's forces are still strong and causing trouble in the realms, even with their god missing.
I can see them rolling out a couple more updated daemons and maybe a couple new sculpts with a battletomb sooner than later, while Slaanesh's escape might not be until 2020 and herald the arrival of the other aelfs.