By storing global operation and throughput thresholds, we could eliminate the "many small memtables" problem caused by having many CFs. The global threshold would be set in the config file, to allow different classes of servers to have different values configured.
Operations occurring in the memtable would add to the global counters, in addition to the memtable-local counters. When a global threshold was violated, the memtable in the system that was using the largest fraction of it's local threshold would be flushed. Local thresholds would continue to act as they always have.
The result would be larger sstables, safer operation with multiple CFs and per node tuning.