。
The problem is what we should do next.
Literal
Problem [topic-は] next [at-に] what [obj-を] should-do [embedded-Q-か] [copula-だ] [という thing-だ].
This sentence has an awkward-looking structure — ~かだということだ stacks か (embedded question) + だ (copula) + という (quotative) + ことだ (nominalizer + copula). This is redundant but grammatically interpretable. In cleaner Japanese, you'd drop one of the copula layers.