I don’t like the idea of ex parte proceedings in general, other than before a Grand Jury.
If you could put together a Grand Jury of citizens who could be granted the necessary Top Secret clearance, I would approve of that. But that would be so cumbersome and unwieldy to setup and manage as to not be practical.
The FISA Courts are therefore an unfortunate necessity. But they could be improved by changing the appointing authority to allow for a more politically diverse set of Judges. Currently the Chief Justice makes 100% of the appointments and the Judges chosen are generally those considered to be more sympathetic to the government.