Some ideas I've kicked around:

Article 1 - I'd start with the Bill of Rights; the rights and responsibilities idea is interesting, but my concern is that a power-mad president or congress could use that to take away rights; I think I'd rather just keep it as things to protect citizens from government

Article 2 - Congress; I've thought of giving the House more of a parliamentary flavor; increase the nominal term to four years, but provide that snap elections can be called at any time by the president, or speaker, or a majority of the House members, limited to one such election every 12 months; elect by proportional representation; push senator terms out to 8 years; also, the one good idea that I thought Jimmy Carter had, choose cabinet members from senators; you'd know in advance who the cabinet was going to be wen voting for president, there would be more accountability than now with cabinet members having to stand for re-election, and we wouldn't waste so much time in confirmation earnings; also, since we seem to have had something of a trend of going from the senate to at least candidacy for the white house, this would give senators a chance to gain some executive experience and us to see them in that role; in this model the senate becomes a part-legislative, part-executive branch; maybe move all bill writing to the house. And do "president's questions" before congress once a month, if not more often.

Article 3 - President; one term, 6 years, cannot succeed himself; drastically cut back the power of unaccountable executive agency bureaucrats; I like the combination of every new reg subject to congressional veto, sunset review of all agencies every 10 years, and moving administrative disputes from trial before an ALJ who reports to the executive director of the agency to separate article III administrative law courts. Perhaps provide for a DC Circuit Court justice to be a 12th member. If you'd tat, to keep from having ties, make the Chief Justice go to the most senior justice and have im vote only in case of ties (which would require a vacancy or recusal of a justice).

Article 4 - Courts; I'd expand the Supreme Court to 11, one per federal judicial circuit, moving to that by appointing two now, one freebie per party, and picking those justices plus future vacancies, from unrepresented districts until every circuit is represented.

Some random thoughts.