Fatboy has some issues. There have been many threads describing how it's fairly weak an ways to change it, so here is my attempt.
One of the main problems is that it's just so dreadfully slow. Not only does this make it difficult to micro and respond to new information, but it also means there is a long time between it getting built and it becoming a problem, since it will take forever to reach the front line.
Problem is, in its current form that speed can't be increased without making it an absolute kiting monster. That's because its turret arcs look like this:
It's just as, if not more, powerful facing rearward and could demolish armies without ever letting them get into range.
This gives us two real options, either remove its ability to fire directly rearwards. or give it a deploy time like non-Aeon T3 mobile arty. The latter might be worth exploring but seems like it would feel like garbage to play, so I will focus on the former.
What if we changed its turret arcs?
This would be a sizable buff directly in front of it, as it can now point all 4 of its turrets at a single target, but it would also mean it can no longer fire while in a full retreat, giving us some options for a speed buff. It would also mean there is a reason to use its ability to back up, which at the moment is functionally useless.
BUT WAIT THERE'S MORE
Fatboy is a combination of a land battleship and mobile firebase, and neither would be complete without TMD. Adding TMD turrets would require some modeling changes, and if previous history has been anything to go by that's been a difficult change to make. However, fatty conveniently already has two (let's be honest, functionally useless) AA turrets. Converting these to TMD should shore up one of fatty's weaknesses as well as giving it an additional unique element as the only mobile land TMD.
Screw it, mobile base? Give it some drones! Either T3 drones like the SACU upgrade or simple kennel drones to help out the factory element or the sparkies it pumps out.
Up until now I've avoided talking specific stat changes, because I think that's really missing the point of what makes a fatty problematic right now. But for the love of god fix that shield recharge time. It's currently a full 2 minutes which is almost 3x that of sera mobile shields.