The best first-party controller on the market at the moment is undoubtedly the Xbox Elite Controller, which has a ton of well-made customisation options that lets you adapt it to your preferences and the game your playing. When it was released, it sold so well it was hard to find in stores for months.
Now it seems like they are preparing a new round of Xbox Elite Controllers and this time with plenty of improvements. This was discovered on Baidu where there's also information about it and pictures. A translation can be found on Resetera, and this is what to be expected if this is true:
Rechargeable batteries, undetachable, MagSafe-like charge port
USB Type-C
Bluetooth on Win10
3-level Hair Trigger locks
3-Profile Switch
Longer key-travel for Paddles
Microsoft has suffered from a ton of leaks recently with games like Perfect Dark, Fable 4 and NBA Jam allegedly revealed, and also a Halo mode based on the Battle Royale concept as well as Forza Horizon 4 taking place in Japan. Assuming some or most of this is true, Microsoft better start working on plugging these leaks if they want to have any surprises left for E3.
What do you think of the potential improvements with a new Xbox One Elite Controller?