Corticospinal tract lesion: Given the progressive nature of the symptoms and the involvement of one side of the body, a lesion affecting the corticospinal tract (which could be due to various causes such as stroke, tumor, or demyelinating diseases) is highly plausible. The normal brain MRI might not have captured the lesion, especially if it's small or in a location not easily visible, or if the MRI was done early in the course of the disease.

Multiple Sclerosis (MS): Although the MRI is reported as normal, MS can sometimes present with minimal or no visible lesions on initial imaging, especially in the early stages. The progressive hemibody symptoms could be indicative of a relapsing-remitting course.
Spinal Cord Lesion: A lesion in the spinal cord, such as a tumor, syrinx, or arteriovenous malformation, could cause progressive hemibody weakness and numbness. The normal brain MRI does not rule out spinal cord pathology.
Peripheral Nerve Disorder: Conditions like multifocal motor neuropathy could present with progressive weakness, although numbness might be less prominent.

Brainstem Lesion: Although the brain MRI is normal, a small lesion in the brainstem could cause significant and progressive symptoms. Brainstem lesions can be life-threatening and require urgent attention.
Cervical Spine Lesion: Similar to brainstem lesions, a lesion in the cervical spine (e.g., tumor, abscess) could have severe consequences, including respiratory compromise, if not promptly addressed.
Vascular Malformations: Arteriovenous malformations (AVMs) or moyamoya disease could lead to progressive neurological deficits and have a high risk of hemorrhage, making them critical not to miss.

Intracranial Hypotension: Could lead to traction on the spinal cord or nerve roots, causing progressive symptoms, although this would typically be accompanied by headache.
Neurosarcoidosis: A rare condition that can affect any part of the nervous system, including the brain and spinal cord, leading to a wide range of neurological symptoms.
Mitochondrial Disorders: Certain mitochondrial myopathies can present with progressive neurological symptoms, including hemibody weakness, although these are typically accompanied by other systemic symptoms.