I think kingdom-building that involves a religious figure would need more than 70k words for it to be done right. However, that's just my opinion.
The slave one feels too cliched, but I can see it fitting into the 70k word limit.
I can see the time travel one could also be done in 70k words, but it's whether the setting in the future would be like the post-apocalyptic one mentioned for the slave one, or not.
(2)
So here's what I think will work: The time travel can be worked through the God herself/himself. That depends on which kind of a God you're thinking of. Is it the elusive Bible kind of God or the joke, Gabriel's Dropout-esque kind of God? If it's the latter, I think you're competent enough to figure out a premise for the time-travelling plot there.
(3)
So they wake up in the future, figured that all went to sh*t and that they have to find a way to get back. As for the slaves and the freedom, I've made something up as I was writing this up:
(4)
In order to return, the God and his/her disciple must sacrifice the slaves. Not just one, but a whole bunch. A damn community. A whole community of slaves who believes that they are free and are willing to trust the God for anything he/she asks for, believing that it'll fare them better in the future.
(5)
Now THERE, you can insert some internal dilemma. Will the slaves come together with the God and the disciple and actually leave an internal impact within the protagonists and cause them to have a split choice between saving themselves or the slaves?