Assuming Battle Fever isn't the only Sentai informing Stacaesar's design, I think an upgrade based on a powerup from that other Sentai, whichever one it is, would work well for him.

I think something based on Abaranger or another Kaiju-like mecha, the latter of which I don't know how many would fit the criteria since I've only watched a couple Sentai seasons

I think Killeroh to go with the dead 6th ranger dino mecha theme.

Yeah, Killeroh-based could work somehow!

But how would it work with Stacaesar's Battle Fever?

I imagine that, since his suit is already purple and black with some white details, they can do something like giving him new gauntlets, boots and torso armor. The helmet is already purple with a silver mouth piece so they can modify an extra one or something to look like the KillerOh

Keep in mind that Stacaesar's base design could change if he ever stops using the dark gears.

Or a different henshin device. Assuming he steals the sixth Geartlinger that debuted in the Zenkai Red special, it might end up being both.

There is no 6th Geartlinger. The Zenkai Red special was all a dream.

Bad news, everyone: Tatsushisa Suzuki has been stepped down as Gege's voice actor. Now, who's going to be the new voice actor replacement for him?

Wow that's unbelievable right.