Congrats to Tadeusz Litak et al. Generalization of first-order logic originating in a neglected work and show that an entirely general completeness result is NOT possible!
Abstract: We propose a generalization of first-order logic originating in a neglected work by C.C. Chang: a natural and generic correspondence language for any types of structures which can be recast as Set-coalgebras. We discuss axiomatization and completeness results for several natural classes of such logics. Moreover, we show that an entirely general completeness result is not possible. We study the expressive power of our language, both in comparison with coalgebraic hybrid logics and with existing first-order proposals for special classes of Set-coalgebras (apart from relational structures, also neighbourhood frames and topological spaces). COninue reading here.
>Litak, Tadeusz, et al. "Model Theory and Proof Theory of Coalgebraic Predicate Logic." arXiv preprint arXiv:1701.03773 (2017).