Unfortunately, eliminating the Senate (or more precisely, each state's equal representation in the Senate) is the one and only thing that the constitution forbids an amendment from doing (see Article V, Section 5):
> Provided that no Amendment [...] no State, without its Consent, shall be deprived of its equal Suffrage in the Senate.
(Awkward ellipsizing, but the elided text is another thing that's not allowed, which expired in 1808, and is otherwise thankfully no longer relevant.)
Better voting systems can be implemented, but since the states run federal elections, each state would have to pass legislation requiring a different voting system. Of course I expect all 50 would not agree on which alternative system is the best, which may or may not matter. And I doubt red states would want to change, as voting systems that better reflect the will of the electorate tend to disadvantage the GOP.
Eliminating gerrymandering is difficult, because it's hard to objectively define what is and isn't a gerrymandered map. There have been some attempts to do so, and I would say they've even been somewhat successful, but people can reasonably disagree with the methodology and thresholds used.
The Citizens United SCOTUS ruling and precedent absolutely needs to be reversed; agreed. Corporations are not people and should not get first amendment protections. Or any kind of protections outside any that are defined in regular law.
Another thing we need to do away with is the Electoral College. Presidents should be elected based on the national popular vote, not by per-state winner-take-all proxies, with vote apportionment that wildly advantages some states over others. The National Popular Vote Interstate Compact would effectively do away with the EC if states "owning" at least 270 electoral votes were to all sign it, but that's unlikely to ever happen. (Then again, it's more likely that the Compact would achieve that threshold than the passing of a constitutional amendment to abolish the EC.)