The thing I always hated about LaGuardia is that it is Taxi accessible only, I could never fathom why the N didn't get extended to resolve that. Other than doing that I think investing in LaGuardia is a mis-allocation of resources.
Several Million People live in Nassau and Suffolk Counties, but to catch an airplane they need to drive past MacArthur/Islip Airport on their way to JFK or LaGuardia through rather intense traffic or take the Long Island Railroad (which may depending on when they're travelling and which line still be slower than sitting in traffic). Spending the money expanding Islip would both add significant capacity for the region and provide a lot of convenience.
Considering New York-Princeton-Philadelphia as a single very large market, Trenton is the most interesting of the Auxilliary Airports. It is not only the shortest drive airport for Central New Jersey (Princeton and New Brunswick), but it is also the shortest drive for airport for the Northern Philadelphia Suburbs including the Northeastern corner of Philadelphia itself and King of Prussia (which is the economic centre of the Philadelphia Market, not downtown). Right now the transit connections to Trenton are awful, but with about 6 miles of track to build there could be direct service from Trenton Airport on three lines to Center City Philadelphia, Camden NJ and New York City. Politically the region either needs to either be taken over by a state or regional authority or get an exemption from the outdated federal restrictions on commercial flights at private airports and be privatized because the Municipal authority that owns it changes their mind every few years as to whether they want to expand it or shut it down entirely.
Because of the tangled politics the money will probably be wasted on a renovation of LaGuardia (probably without the subway extension its always needed and no added capacity or other benefits to travellers than a shiny terminal), rather than on ISP or TNN expansion which would provide much greater benefit to travellers.