I think the changes I'd suggest would be dry and technical, so instead I'll go through the options in the poll, just for fun:
return to 70% tax rate for the 1%
Personally, I don't think it is right for a government to take more than half your earnings. I would like to see some bureaucratic change to close the "Buffett loophole", which I think is well-intentioned but an affront to most people's sense of justice.
mandate universal health/dental care
Yeeeeees... but that depends what you mean. Total coverage of all healthcare needs is unaffordable at this point in time. I would like to see everyone able to access *some* coverage, but "Medicare for All" is not a sensible idea - Medicare is a very generous system.
pay at the pump no-fault car insurance
I don't know exactly what this means.
direct democracy [no electoral college]
Abolishing the electoral college would be an improvement, but is still going to have the issue of Presidency being a fundamentally unrepresentative institution. I would abolish all presidencies and move to a Parliamentary system. Say hello to Prime Minister Pelosi...
mandate national renewable energy infrastructure
Well, yes, but again this is a question of definitions. I'd mandate that the US be carbon neutral by 2050. I do not know when a realistic date would be for decarbonisation of the power sector.
universal education to graduate level
No. Unaffordable, not a public good. Invest that money in primaries and early years.
abolish right to firearms
mandate firearms in every household
I think this should be a states' rights issue. In general I would like more power devolved from the federal government to the states and I think firearms regulation should be part of that. Forcing someone to own a gun is both a bad idea and imo a breach of their rights, but restricting gun ownership is something to be decided at the local level.
humane prison reform, eliminate death penalty
Yes. I'd go further, but not right away.
universal basic income!
Yes.
national tax on sugar in all its forms
I was sceptical but this actually seems to work. Sugar consumption has all sorts of negative externalities and I think it is right that people pay for it.
national sales tax/VAT to replace income tax, with refunds for <$100k income
No, this is a bad idea. It would undermine the tax base and discourage spending. Remove the exemption for "low" earners (someone earning $100k is not a low earner!) and it would be highly regressive.
single 6-yr terms for all federal elected offices
No, good to force politicians to be accountable at the ballot box.
bring our military home to stay!
No, the US military has often been involved in wars that were bad ideas, but it plays a vital peacekeeping role.
stiffen antitrust regs to eliminate "too big to fail" corporate entities
Not sure exactly what the full implications of this would be.
Aside from the poll options, I like Kraftie's suggestion of pay-as-you-earn tax (although important to remember that self-employed people, contractors, and so forth would still need to file returns - but they could be simplified). I also like Drake's suggestions of extra police training, and campaign finance limits.