Which novel immunomodulatory treatment(s) do you consider in refractory seronegative bulbar-predominant myasthenia?

1. Re-evaluate the diagnosis. Acetylcholine receptor and MuSK antibody-negative myasthenia with prominent bulbar symptoms, refractory to all the agents mentioned (steroids, MMF, AZA, IVIG) is an extremely rare scenario. Differential diagnosis includes bulbar onset ALS and numerous myopathies (e.g. IBM, OPMD, mitochondrial, etc). Please note that single fiber EMG and repetitive nerve stimulation of cranial muscles, which are required to confirm a diagnosis of seronegative generalized MG, can be abnormal in bulbar onset ALS, and any other disorder causing rapidly progressive denervation. 
2. Assuming that all the above have been ruled out by a careful physical exam, and EMG, then one reasonable thing would be to try plasma exchange for a few weeks followed by efgartigimod. The mechanism of action of efgartigimod would apply theoretically to any form of autoimmune MG, as it removes IgG antibodies from blood (Acetylcholine receptor, MuSK antibodies, and whatever IgG antibodies cause seronegative MG, which remain to be determined). The problem is insurance approval which in my experience can be very painful- if not impossible-in seronegative MG.

I am not so sure about complement inhibition in seronegative MG (eculizumab, ravulizumab, zilucoplan, etc). It has not been proven that it plays a major role in the pathogenesis, plus those medications are incredibly expensive. 

Non-specific immunosuppressants for MG also include tacrolimus and cyclosporine. They can be very efficient but also quite toxic and they require regular monitoring (BP, kidney and liver function, drug levels monthly, etc).