Not sure what you're saying here. The block rate can't be particularly increased or decreased in the long run due to the work difficulty adjustment getting you roughly back where you started no matter what. Someone could DOS the system by producing empty blocks, sure, that's a central attack of what can happen when someone does a 51% attack with no special countermeasures other than everything that Bitcoin does at its core. An attacker or group of attackers could conspire to reduce block sizes in order to increase transaction fees, in fact they could do that with a miner activated soft fork. That appears both doable and given past things which have happened with transaction fees in the past potentially lucrative, particularly as block rewards fall in the future. Please don't tell the big mining pools about it.