If we had control over the weak force:

• We can likely turn elements into other elements at will
• We can manufacture safe decay sources for a new class of nuclear energy
• We can probably create safe "decay batteries" tuned to their specific use cases. Batteries that last for tens of thousands the lifespan of current chemical ones.
• Potentially engineer with neutrinos. Imagine communication via neutrinos, you could transmit straight through the earth.

I mean, with control over matter like that, at the scale of electricity, Star Trek matter replicators would be a thing.