If your not sure just go for something simple to create a bit of 'glue' between guitars and vox and make everything generally a bit bigger, mixed in to taste. I can imagine a little strings/pad motif working quite well here. Try playing 2 or 3 notes in the chord above the guitar parts i.e. don't play any of the notes the guitar is playing but play higher and it should sound awesome. If you can make a little counter melody to the vocal parts in the keys then all the better as long as it isn't too intrusive.
The vocals held on at the end of each line while the next line starts in addition to the delay has a similar effect to a keys part so you probably wont need very much.