IIRC, MN invested in Genesis engines around the same time the LIRR decided to designed new engines (DMs and DEs) which have turned out to be nothing more than overweight lemons. So for the moment, with the exception of branch line service, there is no need for new motive power.
LIRR Coaches (C3s) are too tall for service to GCT (on either line - as Dutch pointed out, they don't fit on all the tracks along Park Avenue, and they don't fit in the 63rd St tunnels either.
They (C3s) can't be used on low-platforms, meaning that MN would have to either eliminate service to those stops (I think most of the stops left that are low are park stops), waste money on platforms, or go with MBTA style platforms (one car-length high, the rest low). The latter is what the LIRR should have done rather than close stops in Central and western Suffolk (I know they closed a stop or two on OB, but both those stops had very-close alternates)
The only things on order right now are the M7s that are being delivered. CDOT is supposed to order new equipment soon, based on some articles I read in CT newspapers. I think the best thing for them would be a M7-style car that can work with pantographs and possibly offer more seats. Don't forget the bar car!
I have my own ideas about the gear we should see on MN. Look for a new thread discussing this.