I share B.T.'s dislike of previous edition swarm rules, especially 4E. They never really feel like swarms to me; they just feel like singular monsters, sacks of hit points to be beaten down like anything else.
That said, a swarm of 20 singular rats is not something I have any interest in running.
I would prefer to see a swarm treated as an environmental hazard rather than a monster. It doesn't have hit points. Instead, it has squares. Effects that deal area damage above some damage threshold can destroy the squares they target. Non-area attacks have a much higher damage threshold, or perhaps have no effect at all.
Of course, the challenge with this approach would be making it so that swarms are not just another "casters rule, non-casters drool" gotcha.
That said, a swarm of 20 singular rats is not something I have any interest in running.
I would prefer to see a swarm treated as an environmental hazard rather than a monster. It doesn't have hit points. Instead, it has squares. Effects that deal area damage above some damage threshold can destroy the squares they target. Non-area attacks have a much higher damage threshold, or perhaps have no effect at all.
Of course, the challenge with this approach would be making it so that swarms are not just another "casters rule, non-casters drool" gotcha.