It would seem getting this through 27 states would a lower barrier than a lot of other options.
Add in term limits and repeal of the 17th amendment is a better formula then amending the constitution to overturn Citizen United.
As for the repeal of the 17th Amendment, I'm not favor of that, as this is the only hedge we currently have against gerrymandering that keeps our House in the hands of the two parties. There's a reason that there is commonly a party mismatch between the House and Senate. Once gerrymandering is removed from our system, then maybe the Senate itself is redundant in any form, and it can be removed completely.
I also agree with you regrading Citizens United, but it really shouldn't need an amendment since the idea of a corporation existing on its own is a modern day fabrication in the first place. However, since the courts went a bit sideways on this issue, it seems like an amendment will be necessary.
The Senate is only redundant because it is popularly-elected, just like the House.
What it ought to be is the voice of the states. The state legislatures and governors should select senators (I'd like to leave the exact mechanism up to each state, but the key is that the senators should represent the state; honestly, I'd like to make them responsible to the legislature & governor as well).
Our federal system was gravely damaged by the 17th Amendment; the states have become not much more than glorified local governments, rather than the sovereign states they ought to be (and, legally, still are).
[0] https://en.wikipedia.org/wiki/National_Maximum_Speed_Law
There is no need for our state governments to have direct representation in our Federal government. All representatives should be voted upon by the people directly. The idea that the state governments themselves need representation at the Federal level was misguided from the start, in my opinion.
The EU does not act in this fashion because they have strong sovereign states who act in their own interest, and are much the worse for it. When an EU member state gets something like a refugee crisis all the other member states just tell them it's their problem and they need to deal with it. They view monetary policy in the light of which member states it will benefit and which it will harm. They can't even raise an effective Federal military force.
Frankly this type of government already failed as the Articles of Confederation and the Federal system was drafted as a response, but that still doesn't stop some people from clamoring for its return. The Federal system already contains numerous sops to the interests of states, and should not be weakened further.
The House of Representatives already does a pretty good job of representing geographic areas. That's somewhat contrarian since people think of the Senate as the one that's supposed to represent geographic areas (states), but in modern times the House represents the geographic areas (large sparsely-populated areas with conservative membership) and the Senate represents the actual popular vote (nationwide, moderately liberal by a small margin but highly concentrated in urban areas). This is amplified somewhat by gerrymandering, but the truth is that congressional districts are inherently geographic areas and that's the feature the representation conforms to.
If you wanted the House of Representatives to be restored to its original vision, you would need to do exactly what the topic suggests and massively increase the number of representatives to drive the error caused by districting down to a reasonable level. That would be very different from any other large-scale government and would be ruled by informal power structures inside the parties, at which point you might as well just go to a parliamentary system.
While we're on the topic, if I could change one simple feature of US politics it would be for Senators to serve 4 year terms, with every state electing one senator every 2 years. The current system of 6-year terms sucks because parties very predictably dominate presidential elections and off-year elections, and you end up with Senators who are thrown out based only on the fact that they now face an opposite electoral climate from the one they were elected in, that will inevitably reverse on the next 6-year term cycle. There is a huge amount of "churn" here that in my opinion serves no purpose.
Single member districts, first-past-the-post elections, and the Presidential system (which creates a big winner-take-all election, and makes the parties competitive for that election even stronger relative to other parties than they otherwise would be in downballot elections) are what keeps the House (and Senate, you will note, so the gerrymander-proof nature of the Senate is no defense against duopoly) in the hands of the two major parties.
Gerrymandering may effect which of those two parties is likely to hold which seats in the House, and effect competitiveness between the parties for particular seats, but is pretty much irrelevant to duopoly.