glad they were smart enough to ask you, and glad for the smarts/generosity that led you to ask for suggestions.
I actually think the Founders did a fantastic job on the Constitution. I'd nonetheless have several suggestions or areas for tweaking or further research:
The ERA + similar laws protecting the otherly-gendered
Campaign finance reform (ditch Citizens United)
The right of privacy apparently needs to be spelled out
The right of travel ditto
The right to a decent education and health care. In general, cf. what other countries have declared as fundamental human rights.
Powers of and restrictions on the fourth estate. I currently see this a one of the most important oversights in the Founders' work. The fourth estate should function as the watchdog of democracy, not the lapdog of its rich owners. I see no way to fix this except to prohibit direct or indirect ownership of any news media operations by any person or entity that makes any money from or raises money for any purposes other than investigating and reporting news. I immediately see complications that will have to be addressed; but the fourth estate cannot facilitate the kinds of checks and balances needed on the other estates unless it too is subject to checks and balances.
I would like someone to analyze the potential of the 14th amdt to prohibit the state from forcing women to carry unwanted pregnancies to term or to compel organ donation or other assaults on or intrusions into a person's body. I.e., the minute the state can remove an embryo and commits to nursing it to full term, and caring for it or finding someone else to care for it from there on, I'm on board with restricting abortion. But it is clearly outrageous to compel women to subject their bodies to the slavery of an unwanted pregnancy. For that matter, we don't seem to be very clear on whether a person owns their own body, or its DNA, etc. A lot of work needed here.
Related: we need clarification on the degree to which the state can invade our biological and digital minds. Among other things, when should we have a reasonable expectation of privacy (even if Millenials have grown up without any)?
Maybe clarify once and for all that the U.S. is NOT a Christian state,
Ok those are just a few thoughts off the top of my head.