It would have required money. In the two decades the funds per man were cut below just about every other army of the industrial nations. Given the funds here are a few suggestions.
1. Fund a mobilization planning staff. Particularly for the industrial side of mobilization. The US Army was poorly prepared for this in 1939-42 largely because the tiny cadre could not support anything remotely adequate for researching and planning industrial mobilization, or of mobilizing a operational army of more than 250,000 to 500,000. Capability for both this tasks had to be built from scratch from 1939, & took the better part of three years to accomplish the industrial mobilization side.
2. More attention to amphibious warfare. Under War Plan Orange the US Army was to provide a expeditionary force of 50,000 men in the first six months and 100,000 in under 12 months. This was for service across the Pacific. But other than the Joint Army Navy board there was only token participation in developing amphibious warfare. The Joint Board was mostly concerned with strategic, large scale operational, and numbers development. On the tactical & hands on side the Army provided a average of one battalion for amphibious training every 2-3 years over two decades, few liaison or observation officers to the USN training or tests and experiments. For six years from 1932 to 1939 I'm hard pressed to find evidence the Army participated in any joint amphibious warfare training/planning outside the Joint Board.
3. Retain more motorized units during the demobilization of 1919-1923. A example is the Field Artillery was reduced from eight to two motorized artillery regiments, & those were reduced units with the effective strength of a battalion each. Until the conversion of the 7th Cavalry Brigade to a experimental motor/mech unit in the latter 1930s there was no test bed for the theoretical ideas in circulation for mechanized warfare. Attempts to use a infantry division in that era as a test unit were largely ineffectual for lack of funds.
4. Funds to continue development of tactical & "Strike" aviation. The thin funds for R & D in the 1930s went into heavy bomber development. The Army was forced to curtail testing and training in tactical aviation as the 1930s spun out. We had dive bombers & effective close air support in the Nicaragua campaign in the 1920s. Udet observed the US Army aircraft in dive-bombing demonstrations in the 1920s. But, in the 1940s the US Army had to play catch up to everyone else in CAS & did not completely get its act together until 1944.
5. Take a closer look at the rifle battalion weapons. The BAR was obsolete & the HMG/MMG support inadequate. A light MG in the rifle company was badly needed.
6. Make tanks a required part of every training exercise interwar. During WWII regular army & reserve officers trained interwar were inconsistent in their ability to train and operate combined arms formations. While they broke the code at corps & division level the regimental and battalion commanders were not consistently up to speed on training with and integrating tanks into battalion/company combat.
7. Keep at least four infantry divisions at full strength & fully staff four Corps HQ. Training with full size formations suffered badly interwar as repeated funding cuts left the entire army a cadre force.
8. The field artillery shined first & best in WWII, but there was still room for improvement. Lighter radios were possible with the technology of the era, but as with most other things there was not the money to progress the development further. Having the full suite of howitzers, guns, AT guns, and AA weapons tested & ready for production in 1939 would help in the early battles of 1942. All the designs used in WWII were on paper & most partially tested by 1939, but little was actually ready for full scale production
9. Tactical aviation was mentioned earlier. Doctrinally the interceptor/pursuit/fighter arm was neglected & hence development. The single engine fighters of 1939, both in production & being tested were effectively point defense weapons. For a offensive capable army a long range fighter was badly needed. Perhaps larger, more frequent, and better organized training exercises interwar would have revealed this.