Bring back 2 stroke streetbikes for god sakes
This will not happen unless someone develops one that conforms to current and foreseeable emission standards (e.g. Euro 5) ... and that has not happened (legitimately). Yes, I'm aware of a couple of niche products ... I'm skeptical of their claims. Whether they're relying on some sort of exemption for low-volume manufacturers, or relying on some sort of fleet-averaging credit scheme (to allow the high emissions from the two-stroke to be offset by low emissions from a much larger fleet of 4-strokes with full emission control), or simply outright lying/cheating ... I don't know.
Two-strokes with direct-injection work in snowmobile and outboard applications because those aren't subject to emission standards for on-road vehicles.
Technical problems ...
Piston rings require lubrication. They need oil supplied to them. If they cross intake ports, some of that oil inevitably gets flung off and goes into the intake air stream and into the combustion process, where it doesn't burn as well as fuel does. If they cross exhaust ports, some of that oil gets flung off and goes straight out the exhaust port. Catalytic converters don't like getting contaminated by excessive oil and particulate-matter (soot, carbon, heavy hydrocarbons from partly-burned lubrication oil). Detroit Diesel built two-stroke diesel engines for heavy-truck applications for decades, and gave up (and switched to 4-stroke) faced with 1990s-era emission standards - and those standards are a lot tougher today.
Scavenging is imperfect. You can control raw-fuel short-circuiting out the exhaust via direct-injection but you can't control air short-circuiting out the exhaust. A normal 3-way catalyst for 4-stroke engines requires a stoichiometric exhaust stream (the oxidizing part very close to precisely balances the reducing - "fuel" - part). If it's lean (due to dilution from short-circuited air), NOx is poorly controlled. Ask VW how well that turned out for them. Also, the dilution reduces the exhaust temperature which means it will take longer for the catalyst to light up after a cold start. The auto manufacturers already do all sorts of things to intentionally raise exhaust temperature after a cold start to get the catalyst lit up as quickly as possible.
Extreme dilution of the charge by exhaust when running lightly loaded leads to incomplete combustion (high hydrocarbon emissions) even if you can contain the raw fuel inside the cylinder, and even if you can manage to use stratified-charge enough to get reliable ignition (which is very poor with a conventional carbureted two-stroke - the irregular idling is due to misfiring, and it has very high HC emissions when running like that).
The four-stroke engine will be with us until the end of the internal combustion engine and its replacement with battery-electric ...