Great start, and good shots.
I agree with you about the need for a grass solution (like through a Corona scatter capability as in other Corona plugins) as well as the need to for the current plugin to better handle proxies and allow us to have high poly objects like Trees and vegetation and other entourage.
My only criticism of the render (which is more of a style critique than anything) would be a suggestion for you to avoid doing what a lot of other people do a lot of (and essentially abuse) in their renders.
Which is, don't have the lights or lamps in your scene turned on when you're doing a daytime render with the sun high up in the sky.
Aside from the fact that you literally wouldn't do that in real life, it also detracts from the realism of your render because there's no way, given how bright the sun really is, that you would see those light washes on the wall on the exterior and on the same wall that is also being illuminated by the sun.
It makes your render look fake and more CGI than it deserves.
Same thing could be said about the lights on the inside as one wouldn't see them lighting up the interior that bright relative to the sun.
Even with lights turned on, in the middle of the day an interior space (with big enough windows and openings) will still get most of it's lighting from the sun's light rather than from the lamps or lights inside.
Some of the textures and materials could also use some more work (or you could use more AO it mask the "plasticness" and fake some dirt and wear and tear), but other than that,...
not too bad for a start.
