A sermon should have high degree of eloquence
هُوَ أَفْصَحُ مِنِّي لِسَانًا (He is more fluent in his tongue - Al-Qasas 28:34 ⧉). This verse Al-Qasas 28:34 ⧉ points out that a high degree of oratory and eloquence is desirable for sermons and preaching, and there is no harm if one takes training in that.