Yeah, riff based songs are hard to take in another direction, without it sounding forced. The good thing is that if you have a good rhythm section, you can just ride that riff, and no one will mind. Really you can't stray from the I-IV-V loop, unless the change is just brilliant. There's always the early Zeppelin method: bad ass riff, spacey freak out section, different bad ass riff, Robert Plant orgasm, back to first bad ass riff. The hard thing about songwriting is there really aren't any rules, except don't write something that sounds like shit. You could listen to 100 songs, and there may be 100 different reasons that they work. Some are just masterful gems (Lennon/McCartney, early Bowie), some groovin' riffs (Zep, and the blues guys they ripped off), some lyrical content (Dylan), some lyrical message (Marley), some for the magnitude of sound or production (Pink Floyd, RATM), some are throwaway songs, but feature an great instrumental (Hendrix) or vocal (Aretha) performance, and on and on. Likewise, you could play the same song for 100 people, and likely a third will love it, a third will hate it, and a third could care less. The hard part is, once you've got a song to take shape, it's nearly impossible to listen to it with fresh ears.
My theory background comes from jazz (bass), so it's pretty comprehensive. Still, I'm constantly impressed when I'm learning a rock song, and there's a chord change that blows me away, because it goes against the logical conventions of tonality. Bowie's really good for that. There are some songs (not necessarily Bowie) that are seemingly a random collection of major chords, regardless of key, yet they sound great. I just think that's cool. It is interesting how most of the top rock songwriters are at least casually aware of the "Tin Pan Alley" style. Usually, if I'm trying to come up with ideas, I'll just strum my guitar and try to zone out, just playing random shit until something catches my ear. I'm not really good at translating what I hear in my head to what I can play (though I can usually pick out the root motion), so a lot of ideas just fade in to the ether.