As you and the Congress have made clear, your early focus will be on the ACA--a program that has remained unpopular with the public. Defunding major portions of the ACA will be possible as long as there is a simple majority of support for it in the Senate. The challenge will be to build a consensus on a replacement that will cover about as many people as were newly insured under the ACA--about 20 million although it may have grown during the latest enrollment period.
The majority of the newly insured were on Medicaid and with all its limitations, the Medicaid expansion seemed to work pretty well. Of course, that was when the Federal government was paying 100% of the expansion. That's about to change. The exchanges have worked less well. Big swings in premium prices, many insurers leaving because of continued losses, some frustration with coverage chosen, etc. Various policy decisions made that predictable.
Decisions about the structure and funding of Medicaid will be important since that's where most of the newly insured are. Replacing the funding and rules for the exchanges will also be important. Unintended consequences are common. Rules regarding insurance will require 60 votes in the Senate--a bipartisan coalition.
The VA Choice Act and the CHIP program also need to be reauthorized, allowed to expire or changed. There is plenty of indication serious problems remain at the VA. Changing that culture is tough...
Fixing/replacing the ACA is important but not your only health care challenge.