Kind of a general take but I think a lot of countries are way over-focusing on stealth technology. Assuming a detection system is developed which does not rely on sending out radio waves and then picking up on the ones that bounce back, stealth aircraft could become effectively worthless overnight. Keep in mind that not only has a ton of money been spent on stealth tech, a lot of the new generation 5 fighters have greatly sacrificed performance in other aspects in order to accommodate them being stealthy. Something I was thinking that fits the bill of a stealth defeating technology would be a detection system that detects micro alterations to the Earth's natural magnetic field to detect aircraft. This is not at all something far-fetched since while it is in its infancy currently, such technology does already
exist. Who knows whether or not a country is actually working on this, but if a rando on the internet like myself thought of it, there's a fair chance that some expert military engineer has also thought of it.