Modifying buckets (and cells) to not being able to carry up lava (that would cause the bucket to melt and a lava block be placed right on you) would be somethin good to balance.
You would need metals that melt over 1000K, like aluminium.
I'd like the bucket-melting(burning?) time delay to depend on the properties of the bucket. E.g wooden one would burn near-instantly, aluminum in a couple of seconds, iron half a minute etc.