Add the top & bottom ground polygons. If you have to cut a trace and add a wire, the ground plane will not make a difference. You could lay the 7805 flat on the board than and use the ground planes to help cool it.The design needs some more caps - on the VCC, AVCC, AREF, and 22 pf caps on the XTAL1 & XTAL2 pins.I see an ICSP connector for downloading a bootloader, or a sketch that starts in place of a bootloader.I see a serial interface if you want to download a sketch that way via the bootloader. The pinout used is not standard. You might want to revisit that so you plug on a standard part like an FTDI cable or an FTDI Basic. If you're planning on making your own adapter cable you're all set.A Diode across the pullup resistor will make for more reliable downloads,
Is that a RS232 chip for serial conversion? If so, usually there are some capacitors involved for it to work properly.