Use hymns as openers. Whether as a Call To Worship or as an Invocation, the beginning of a service is a good place to use a hymn. Since this song stands alone, there are no flow problems. I suggest that the key be well chosen so that the next song flows from the key of the opener even if someone prays in between. Use hymns as closers.
Hymns can be great wrap-up songs.
Ending with something familiar is always strong. Thematic links can be made with contemporary songs if you flow out of a new song into an old song that says the same thing. Use the same hymn as both the opener and closer. The significant repeat of a song can be most effective when the worship set has a theme. Many of my orchestrations of hymns are designed to be used as openers or closers.