While sadly no one in positions of power are going to look at this issue seriously (if at all), I will follow Chris Pook's lead and try to do a bit of catalogue shopping.
Perhaps the most reasonable alternative might be the Dragonfire 2 system trialled by the USMC some years ago. A 120mm mortar system which is capable of automatic fire and is highly automated, so only a very small gun det is needed per tube. Dragonfire can be carried on a towed mount, but was also demonstrated mounted in a LAV 25 chassis, so there are two options which fit with out ways of doing things (a Dragonfire 2 in a re-purposed LAV III chassis or a LAV 6.0 would be no problem.
Thinking a bit sideways, the Russian 82mm 2B9 Vasilek automatic mortar has been trialled mounted on the back of a HMMVW, as well as in Russian service ion Afghanistan on the rear deck of BMP's and MTLB's. While the Russian mortar itself isn't going to be in our inventory, there seems to be no reason that it can't be "reverse engineered" by us. The 2B9 Vasilek also comes on a towed mount, so our analogue can be pulled by a light or medium truck, if desired.
The Cockerill CT-CV 105HP turret is a "drop in" turret for LAV class vehicles. While designed for armoured warfare, it has the unusual property of begin a Wegmann turret with a max elevation of 420. Armoured troops have the ability to use their cannons in a "semi indirect" role, and firing 105mm rounds from a mobile, armoured platform is always a useful ability, even if the cap badge is armoured rather than "Unique". Similar abilities exist with single and dual barrelled AMOS 120mm mortar turrets for LAV class vehicles.
Finally, the Swedish "Archer" SP system provides a 155 cannon with a high degree of mobility and autonomy. Each gun is mounted on an articulated truck chassis and only requires a 3 man Det to set up and fire. The gun fires form a magazine, so the echelon follows with pre loaded magazines for quick change outs, also using largely automated systems. This is the most capable of all the catalogue choices, and also the most expensive in terms of unit cost and the amount of second line support that it might need, but it compliments the M-777 in both range and ability, and while the M-777 can be moved by helicopter and road, the Archer is readable and has some cross country mobility to compliment the M-777.
While Rocket artillery and GBAD/C-RAM systems are also important elements of the Artillery park. I would suggest they are more for the Reg Force than the Reserve.