If you want to explore the 'experimental' side of things as Jip was mentioning, maybe give the Fatboy 10 or so 'slots' where he can 'transport' (equip) units, which could still work when 'mounted'. Say it can equip 2 Parashields, 4 Mercies and 4 Cougars - then it becomes a mobile fortress equipped with additional shields, fire-power and very strong anti-air.
Or give it staggered fire, higher arc, larger scatter and larger damage radius - effectively, a small-scale Scathis. The better scatter + damage radius will make it harder to dodge, though you might want to nudge it's alpha then so it doesn't just wipe non-experimentals from existence.
Or just turn it into a MAA experimental, and have it's main guns able to fire at air targets lol.
Actually you could give it like, in addition to being able to produce T3 land, the ability to produce up to T2 air as well. If paired with I hope soon coming fix to the mobile factories working even when on move, you could churn out some bombers or gunships to help it against incoming direct-fire experimentals.