Reasonable speculation starts at a point where science will not go, i.e. like the question of what started the expansion of our observable universe.

Reasonable speculation starts with a step by step description of what might have been the circumstances at the point that expansion began and builds step by step to the point that we observe, i.e. the galaxies moving away from each other.

Many of the steps might agree with existing theory but since the existing theory doesn't address the start point, speculation is needed and new physics has to be predicted if any explanation is to be offered.

Reasonable speculation is subject to peer review just like scientific theory, but the peers are only those who care to make their opinions known in public which often excludes qualified professionals .

Reasonable speculation can be objected to by peers, but those peers must offer an alternative speculation. Reasonable speculation then changes as better explanations for unfalsifiable events are developed.

Advancement in science can also affect reasonable speculation. If a unifying particle were to be discovered in particle physics then speculations that didn't include such unification would have to be updated. If evidence of the existence of a greater universe beyond our expanding universe were to become evident then speculation would have to change to include that, etc.