Down East Microwave has announced a 5 band transverter which appears to be idea for use with a Flex-6700/6600 transceiver.
Description given is:
[A] 5 band transverter covering all amateur radio bands from 2M through 23 cm. All frequencies convert to/from 28 MHz. The transmit output level is approximately 1 watt and will exhibit a 2 dB nose figure with greater than 15 dB gain on all bands. The Transverter will also contain an AUX RF port that will be configured as an additional 2M port for connection to a higher frequency transverter such as our future DEM MICRO-VERTER containing 4 additional higher bands of operation. But as an option, the AUX port could be configured as a 4M port or any other frequency within the range of the transverter.
The DEM V/U XVERTER -VHF/UHF Multiband Transverter info sheet is at http://01895fa.netsolhost.com/PDF/V-UXVERT_pd.pdf and the product page link at https://www.downeastmicrowave.com/product-p/vuxvert.htm
To be effective amplifiers for the bands will be required.
Interesting and worth watching.