Your EGR system already does this by keeping or rerouting exhaust (mostly CO2) back into the cylinder. The problem is that at low loads too much CO2 results in misfires for most vehicles, which results in other emissions spikes. The newer GDI engines allow for precise control of the incoming fuel mixture, which lets manufacturers run more EGR and relatively lean A:F ratios in order to improve BSFC w/o increasing emissions.
Here's some more info.