Externally, existential there is parsed as if it were the subject of its clause, so there is the subject used in tag questions; and the verb is the tensed auxiliary which heads the main clause:
There would be, wouldn't there?
There will be, won't there?
There has been, hasn't there?
There was, wasn't there?
In those cases where there is no auxiliary in the main clause, DO is used in the tag question:
There followed three examples, didn't there?
But such situations are very rare; tag questions are mostly confined to colloquial registers, and you don't often encounter there with any verb but a BE construction in improvised speech.