Just as you can omit 7ths entirely for a more modern sound, you can also overuse them.
Try playing all chords in a song as 7th chords. It doesn't suddenly make a song sound entirely like a folk song. It also doesn't suit all types of music, but it can make a song sound more bluesy.
Our example song would then look like this:
[C7]Tra-la-la-la- [F7]Laa.
[G7]Tra-la-la-la- [C7]Laa.
Or, why not play a blues exactly this way. In fact, it's not entirely uncommon – it truly makes a blues sound... even more like a blues.
