I don't see why there wouldn't be - nothing in the rewards previewed so far stands out as a "Frontier will absolutely expect everyone to have access to this, which is only obtainable through Powerplay". The only PP-exclusive things at all shown so far seem to be modules, even the best of which aren't essential.I'm hoping, but seriously doubting, that there will be some way to avoid pledging to any power; I'm more dedicated to the pilots' federation than anything or anyone else. Would love for that to be a viable path, it just makes sense for a bounty hunter to be completely independent of any political beliefs and their inherent ties.
I suppose it depends how you view something like ALD's bounty hunting bonuses in PP1 - getting double pay is a powerful bribe, but on the other hand it's certainly not compulsory to accept it.