It's no Secret the Siege tank sees little use in the more Advanced Matches, and is currently underwhelming for its position in the Tech tree, and requiring its own factory, the Fact that this Unit could use to be better is well known. The Current Role of the Siege tank isn't really on the table for changing, it's weakness to swarming and inability to fight aircraft is intentional while it is meant to Reign supreme in Vehicle and Structure Destruction.
Important note the Current Siege tank will be renamed the Plasma Tank and will be Made exclusive to Sol when Centauri are added, who will get their own Siege tank Resembling the Command and Conquer Mammoth tank. Keep in mind this is a list of Potential Buffs adding all would not be Recommended
Combat Buffs:
Increased Deflection, making the Siege tank the god of deflection I think would work well to improve it's survivability allowing it's firepower to shine. In a Head on Confrontation Rail tanks should have serious Issues combating the Siege Tank's Deflection, this will increase the need to counter using flanking, aircraft and Particularly for Aliens closing the Gap and using Melee. Keep in mind these weaknesses are meant to be covered for by supporting units, the Siege tank is not a do all Vehicle.
Fire Patch, simple enough the shot of the Plasma Tank landing would cause a fire Patch from superheated gasses that will damage any unit walking on the spot, this lasting damage will increase it's performance vs structures while giving the Siege tank the Ability of Area Denial.
Indirect Fire, adding gravity to the Plasma Ball could allow players to use it as indirect fire potentially hitting enemies hidden over Cliffs and Sand Dunes. This comes with costs that would be more of a sidegrade, and requires supporting UI work.
A Charge up Mechanic, allowing the player to charge the Plasma Ball may be a simple way to improve his range and Damage while Lowering the Rate of fire.