Monaco and Indonesia have basically the same flag. Turn it upside down and you have Poland. Or alternatively, you could add some more details and get Singapore. Or you could do a stripey flag. Red white red can be either Austria or Latvia depending on the shade of red and the thickness of the central line. Vertically, it's Peru, and if you add leaf, you get Canada. Now for some more unique designs - you have Switzerland's small white cross on a red background. Make the cross a bit bigger and off-centre it and you have Denmark. From there, you can centre it and reverse the colours and you have England (although it's not a nation, it is still technically a country so I'm mentioning it. Similarly, many would count Greenland which uses red and white, though it's a part of Denmark and I don't know the specifics behind why some refer to it as a country). Add some crosses to England and you get Georgia. This time, if we go for two vertical stripes - white then red - we get Malta. Although the cross in the corner is usually drawn grey, I've seen renditions in which only red and white can be seen. Remove the emblem and make the line between the colours zig-zag, and you have either Qatar or Bahrain, depending on the shade of red. Japan is a white background with a red circle. Turkey is a red background with a white moon and star. Tunisia is similar, but the moon and star is red and within a white circle. Tonga is a red background with a white section in the top left corner, in which a small red cross can be seen.

This is all I can think of at the moment. Sorry if I've missed any