Dear sir,
Any anterior mediastinal mass needs a thorough evaluation. From the description you have provided it could be any one of the things you have mentioned: lymph nodes, thymoma, GCT, etc.
Next Steps
I would suggest you to consider a few blood tests: AFP,
beta HCG, LDH. Also get a image guided FNAC or biopsy. This would narrow down the diagnosis and then a treatment can be planned.
Health Tips
if it is a thymoma or thymic carcinoma, a thoracoscopic surgery can be planned, which would be the most painless option for the patient.