I'm thinking some weapons that we still make, and take for granted their usefulness, are going to become obsolete in the next big war, such as horses became obsolete by the end of WW2. I'm thinking about tanks, ships, helicopters, maybe even rockets. To make it clear: I hope we are smart enough to not allow the next big war to happen.
For example, how would I neutralise a tank?
A small (5-10cm) flying drone with a small hardened part, finds a tank and lands inside its gun barrel. From here I see two possibilities: it could latch itself to the inside of the gun to prevent firing of the gun or it could wait until the hatch opens and then get inside the turret to neutralise the crew. That's it, either no explosives, just a small, hard part, in a place where it shouldn't be or a small explosive detonated when it would hurt most.
The small size of the drone protects it from being detected and destroyed. It could even fly in a zig-zag fashion like a mosquito or a fly - it does not have to be particularly fast.
How to neutralise a helicopter? Maybe make the drone land on its tail-rotor and either jam it or damage it by a small explosion. In general, use small size and great manoeuvrability to attack where the helicopter is most vulnerable.
How to destroy fast moving objects like airplanes and rockets? By using something that is ultimately fast: laser shoots at 300,000 km/s. Calculating the angles and firing is an ideal job for a radar/computer. How to detect stealth planes? Maybe by creating a virtual net from solar/battery powered radar/camera drones permanently patrolling the sky. If some of them had lasers on board, the system might be even more effective.