Prediction for 2019:

Free verse and formal, narrative to confessional to modernist styles, interesting themes, striking images, differing viewpoints, depth of insight, the will to write and think - all are important to the poetry-minded.
In early Christian centuries, there weren't any, only plainchant, of
which 'O Come, O Come, Emmanuel' is a remnant, though it's most often
sung at a faster pace and the long notes are cut short.
Later,
the church relaxed its grip a little and allowed adapted folksongs and
specially written carols into the canon in the Cs.15-16th - and
in churches; 'Personent Hodie,' 'Balulalow' and 'Tomorrow shall be my
Dancing Day' show something of the differing traditions. It was around that time that the verses in carols became metred and rhymed, probably because they were easier to fit into a particular and much liked tune and harmony. Carols proper
were for dancing, too, or for at least processing around houses, fields and
orchards; a few were lullabys, 'The Coventry Carol' being one. Some of
them had pagan origins; the Wassail carols and 'The Holly and the Ivy'
are examples.
Not all carols were for Christmas; New Year, Easter,
Mayday, Harvest Festival and more Church festivals were celebrated. The
Welsh carol 'Nos Falan/Deck the Halls' is a New Year, not a Christmas
carol. Then there were carols based on folk legends or extension of scripture; 'As it Fell out on a Bright Summer's Day' is one of those, though it's not very well known except among folk singers.