The NW Digital Radio (UHF 56kbps etc) high speed UHF data radio: proposed UDRX-440 has been scrapped. (Initially announced in May 2012)
Jerry at Connect Systems hasn't been able to get the needed cooperation from Co-Value or any other hardware manufacture for that multi-protocol Digital Voice radio, the proposed CS7000. (Initially announced in May 2014) So that's in hiatus till someone can design the hardware.
That DV4 Mobile that the German Wireless Holdings guys showed at Dayton 2016 has ran into redesign/parts issues, so that appears to be going nowhere too. (Initially announced July 2015)
Then there is Bruce Perens who was/is working with Chris Testa (KD2BMH) on some sort of SDR based VHF/UHF TDMA radio (Algoram, Katena, Whitebox - first talked about in April 2015) The inital idea was likely too ambitious, and they ran into RF problems.
If you need to review these endeavors, Gary KN4AQ made a reddit post a while back that goes into a little more detail
And from Bruce's 2017 DCC talk, he details some of the snags. Here are some relevant extracts:
Next Step, First Try
Chris Testa and I tried to build a power-efficient SDR HT with a radio based on the CMX991 and a computer with built-in FLASH-based gate-array based on Microsemi SmartFusion.
Chris and I spent a lot of time making the computer. By the time we were done, we could buy better, faster computers, already built, for less. We won’t make computers again.
I bought a lot of test equipment at surplus, so we each have a pretty good lab.
Chris and I got the computer working before we entirely debugged the radio. In retrospect this was backwards, and we should have built a radio that we could debug without building a computer at all.
To make up for the computer’s low speed, we took too long working on gate-array code.
The radio design turned out to be too noisy, and that killed the design. By the time we got to that point, there were a lot better platforms than CMX991.
Why Not Use Raspberry Pi 3?
Many small, powerful, and really cheap computers, like Raspberry Pi 3, are too I/O limited to do high-bandwidth SDR. In the case of Pi 3, its USB 2 is too slow, and it has serial channels dedicated to a camera and display that might have worked, except that they aren’t fully documented and depend upon undocumented coprocessors.
But there are somewhat more expensive boards with USB3, etc.
Chris and I took a two-year break to work on other things after this design failure. In that time, nobody has approached creating the radio we wanted. So, it is probably time to work on the next version.
This would use an existing computer, existing SDR board, and only require the production of hardware for filters, amplifiers, and glue.
I have the development hardware on hand.
I guess the Runbo line of Android phones with an integrated two way radio chipset is worth mentioning. Some people think that thing might evolve into something. The problem is I have been keeping an eye on this longer than all of the above, perhaps back to 2013. In that time I haven't seen much. I'd be surprised if whoever makes these things would be willing to work with anyone to let them access the development side of things so that a potential reseller could investigate if the hardware/chipset is capable of doing much outside of analog FM etc or if there is enough resources under the hood to do multimode digital.
And for anyone who is still disillusion enough to think one of the big three is going to develop this multimode radio, wake up! It would have happened by now. While I tend to agree they would be in the best position in terms of engineering and assets, seems they are not interesting in competing. They are still interested in locking everyone into their digital flavor.
I'd still be happy to throw some money into a gofund me sort of thing to get Jonathan G4KLX to code a client/user end type of MMDVM application. Where you use a AMBE dongle or sudo dongle for that part, and interface it all to the same type of Arduino interface that his repeater MMDVM interfaces uses to connect to analog radios. The problem is he doesn't have the free time like he used to.