For me, a form with some relation to the subject matter is always ideal, but I tend to fall in love most easily with ghazal, sestina, pantoum, and villanelle. There's something empowering about well-placed repetition, like a mantra or prayer.
I've never heard of heteronyms before, they sound so interesting! The idea of writing not just under assumed names but assumed personalities must be so much fun I'm definitely going to look into Pessoa's poetry. What about heteronyms appeals to you (for example, the deliberate creation of another personality, or perhaps seeing shades of the original author in the created personality)? Do you find it differs a great deal from poetry written by poets who write as a character other than themselves, but not with the deliberate intention of creating a heteronym?
Epic poetry is a lot of fun, I really like poems that tell stories What type of heroes do you like to read (and write) about? (Do you prefer fantasy, historical, mythological, etc.?) How long of a poem would you consider a mini epic, as opposed to a true epic?
I found a few of his poems online, I especially like the way he ends some of his lines, almost tricking the reader into assuming he means one thing until you read the next line. What draws you to prose poetry? Do you find it easier to read than fixed forms, or perhaps more coherent? Does the subject matter enter into the equation?
Before I'll reply to your question, I'll have to ask you something first. Have you read prose poems by Hass? If you talk about line breaks, I'd say you've read his stuff in free verse. A prose poem is a poem written as a piece of prose, so without any linebreaks or something. Not meaning to bitch on you or something.