Heaven - Season One by Mur Lafferty

What if Heaven wasn't all it's cracked up to be? Friends Kate and Daniel find that after their untimely deaths, Heaven leaves them dissatisfied and itching for something... else. So they're off, with a passport to discover more afterlifes, heroes and gods. During their adventures, they find out that their travel isn't a journey taken on a whim, but may be orchestrated, or even prophesied.