To solve the big problem of inability to channel near the Pit of Doom, a non-channeling weapon would certainly be needed. From the flow of the books, it appears that only Mat would be in a position to invent new weapons of that kind.

Besides, a new secret weapon is always a morale booster, as it can give people hope that some additional secret weapons are on the horizion.

So, in a way, I can see Mat spend most of his time in the Mad Scientist's lab to create new weapons, instead of planning battles.

There are several ingredients already present to make new weapons.

Recipe 1:
To'raken + Grenades + Strikers = Bombers.

Recipe 2:
Steam Weagons + Dragons = Self-Propelled Artillery.

Recipe 3:
Steam Weagons + Cuendillar Armor + 4 cross-bow men = Tanks.

Surprises like that would work better in the flow of the series.