The M109A7 is a good weapon but the 155mm Howitzer has a range issue, with a RAP round it can only go to 40 kilometers, that's not particularly good by any standards. Other nations, like the South African G6, French CAESAR (CAmion Équipé d'un Système d'ARtillerie), and the Panzerhaubitze 2000, all outrange the M109's 155mm howitzer.
There is a plan to upgrade the tube of the M109A7 from 39 caliber to 58 caliber. However, I don't know the specification on that weapon. Some estimates put its range at 70 kilometers (well over 43 miles) but:
a. Tube wear is an unknown.
b. Will a new recoil system have to be built and if so will the Army ask for an upgrade to a possible M109A8 Standard.
c. Can the hydraulics quickly lower and raise the new barrel (tube). Will an upgrade be needed. See the A8 upgrade question.
d. Can the upgraded/re-tasked Bradley running gear take the shock of repeated firings at the highest charges for range. Already the Bradley running gear carries near 40 tons. See the A8 question.
e. Can the upgraded/re-tasked Bradley running gear take the stress of a 58 caliber barrel in near 40 MPH movements on a constant basis. Are we now talking about a A9 type upgrade?
Note, "e" is a big deal. The German Panzer IV F Ausf 2 was thought to be a fair answer for the Soviet T-34, but suspension issues meant there were always maintenance issues. Everybody thinks German tanks were fantastic in WWII until the subject of maintenance comes up. If the damn thing is broke then what's the point?
All the WWII fans will attack the Sherman M4 for being a poor tank. The Germans thought quite well of the captured tanks. Why? They were easy to work on and reliable.
Military combat vehicles are like combat ships, the whole design is a balance and a change to "A" can drastically effect "B". Never forget that economics make a huge factor in peace. Because of economics the US Navy got rid of the F-14 and that decision is regretted to this day.