a) The power of Tefilla. There is so much to say, and so many unbelievable stories, any Drosho can be a winner. Especially to those who don’t hear the same thing every year. And if they are the sort of people who only come once a year, you can emphasize how HKB”H loves every person and all their tefillos…
b) Malchiyus – the focal point of Rosh Hashono is to remember that G-D is our King, and we must be Machniah [humble] ourselves to Him. Of course mention Yiras Shamayim, and why we don’t say Viduy on Rosh Hashana… and so on.