So, we've got to figure out Garfield's position on things first off, then figure out what was actually possible for him to do at the time. Going off the shortlist you've provided:
- Economics
Garfield supported the Silver Standard and wanted a fixed silver standard system worldwide. He also wished for a reduction in interest on debt.
Called for more investment in agriculture, as well as in developing industries.
Supported the construction of a canal in Central America to be controlled by the United States.
- Reform and Regulation
In addition to ending the practice of patronage, Garfield also wanted term limits on minor Executive Department posts (exactly what 'minor' means in this case is not clear to me, that is the wording he uses).
- Military
There is little information on Garfield's political military stances, and the same goes for his Secretary of War, Robert Todd Lincoln.
- Domestic Policies
Garfield was critical of Mormons and hoped to enforce the abolition of polygamy in Utah.
Advocated literacy programs, especially in the South, and called on Congress to implement universal education (whether this means higher education, or simply primary and secondary education is debatable).
- Civil Rights
Garfield was strongly opposed to the Jim Crow laws of the South.
- International Affairs
Garfield was very interested in increasing relations with Latin America and the developing Republics. (Remember this was before the Spanish-American War, so Spain still had some influence in the Caribbean)
- Supreme Court
Garfield appointed only one Justice to the Supreme Court in his term, Stanley Matthews, a lawyer and politician who served on the Supreme Court only seven years.
Nevertheless, Matthews was influential on at least one potentially relevant case: Yick Wo v. Hopkins, a case in which Matthews, writing in the opinion of the Court, established that laws which are administered discriminatively based on race were unconstitutional, no matter the intent of the law.
- Future Elections
Within the nine months of his term, Garfield effectively united the disparate Republican factions under him, culminated by the resignation of Conkling, his largest Congressional opposition. He was adept at choosing political appointments in a manner that balanced political interest or at least tended to satisfy everyone enough that he remained popular.
My bet would be on him winning by a much greater margin in the 1884 election, based solely on what he was trying to do at the time of his assassination, not even considering what he was capable of doing.
FYI, most of the information provided here was based on his intentions as expressed in his inaugural address.