It didn’t appear to me that their staples of warfighting in WWII were ever lacking. The Gewehr-43, Panzer IV, V-1 rocket, etc. all seemed to do the job well enough without the need for their successors. In fact, the only weapon that appeared to need improvement was the U-Boat, but the need for that improvement wouldn’t show itself till halfway through the war.
Thanks for any replies.
You may be getting an over-representation of German "secret projects" thanks to pop history. Thanks to Germany being ovverrun at the end of the war, all their factories fell into Allied hands, and with them came all the prototypes and design drawings that normally would be a long way off from service. A couple areas stand out - particularly rocketry - but in many areas, German weapons development largely seemed to parallel that of the Allies. The big difference is that, while Allied "secret projects" largely ended up never leaving the drawing board or testing grounds, German ones fell into Allied hands and became more publicly known.
The particularly notable instance is with aircraft. Having helped sort design drawings from Martin's archives myself, I know for a fact that there were aircraft projects on the drawing board just as absurd and outlandish as a lot of proposed German designs. My favorite was the design studies leading up to the XB-48, which included a canard and twin-boom design. However, with all of these Allied manufacturers, the design studies that never materialized ended at that, and while they weren't exactly secret much of the time, they just sat in archives until someone dug them out decades later. Germany, on the other hand, had the (mis)fortune of having most of their design offices consciously sacked for R&D work at the end of the war and many of their scientists and engineers brought back to the US or Soviet Union. Because of that, a lot of their R&D work became more widely available in the public domain.
Outside of aircraft, I'm not as well versed in things, but quite a bit of their development wasn't as ambitious as it may be portrayed - it was just keeping up with the Allies. The infamous MP43/StG44 is hailed as the first assault rifle and 8mm Kurz as the first modern intermediate cartridge, but the Soviets were converging on similar solutions around the same time, developing the 7.62X39mm cartridge (most famously used in the AK-47) in 1943 and possibly even deploying the SKS in the final stages of the war. The Panzer IV on paper seemed fine, but in reality it was at the end of its useful design life and needed a full-fledged replacement. The King Tiger and other super-heavy tanks are often portrayed as absurd wastes of resources, but the Allies had similar designs in active testing by the end of the war (T29 Heavy Tank and T28 Super Heavy Tank for America, IS-3 and IS-7 for the Soviets).