  In this thesis I explore the generality of the holographic principle in 2+1
(bulk) dimensions by looking at the possibility of having holographic
correspondences for non-AdS (higher-spin) spacetimes. The first part focuses on
Lobachevsky spacetimes with $\mathfrak{sl}(4,\mathbb{R})$ as well as
$\mathcal{W}^{(2)}_N$ symmetries, the asymptotic symmetry algebras and their
unitary representations. This results in a family of unitary
$\mathcal{W}^{(2)}_N$ models that can have both small and large central charge.
The focus of the second part is a possible holographic correspondence in
asymptotically flat spacetimes. This part covers limits from known AdS$_3$
results to flat space as well as a NO-GO result that forbids having flat space,
higher-spins and unitarity at the same time. In addition this part shows how to
consistently add (higher-spin) chemical potentials to flat space. As a
non-trivial check of a holographic correspondence in flat space I provide a way
to determine entanglement entropy (as well as thermal entropy of flat space
cosmologies) holographically in asymptotically flat spacetimes using Wilson
lines.
