Three suggestions here. First, could the Lantern Shield be incorporated into the Labrynna Regime, perhaps utilized by the Homeguards (as perhaps an upgrade) or perhaps even duel wielded by mechs? It was essentially a fusion between a buckler, a gauntlet, a sword, and a lantern.
The second suggestion is a modified version of the medieval gun-shield. Just as it sounds, its a fusion between a firearm and a shield. In this case, it could perhaps be modified for a more ornate design, and perhaps made into a tower shield for increased stability as well as allowing a larger gun to be built into it.
Finally, my third suggestion is incorporating the handheld cannon utilized by Aaru to retake Hyrule Castle towards the end of Twilight Princess. Perhaps a larger version used by a secondary mech unit, under the idea that they're the only ones strong enough to carry/use it?