Corrugator muscle loss after unilateral bleph & ptosis repair (12 months after)

Good question. A photo would be helpful. First, I would consult with your original surgeon. It depends on the problem; however, it sounds like a reasonable solution based on the description of the problem is, as you suggested, HA filler. If you like it and the problem gets fixed, repeat injections or perform fat grafting for more permanent results. Again, it depends on the situation; sometimes, a botulinum toxin on the contralateral side could make things symmetric.