User blog comment:TheStudyOfTheUnknown/A hint to the 1.9 vanilla update/@comment-26222246-20150403211002

I do not know what exact features will be added but they will be combat related,

here are the only confirmed features:

Ender dragon will breath plasma and some of the health regenerating blocks will have iron bars around them

you will be able to take 1 passive nmob with you in a boat (except horses and bats)

Multiple boss bars can be displayed at once

A strange dark block type

A boss bar that said Other Wither: Possibly another wither boss type