There is (experimental?) support to store historical data on Cassandra (https://support.zabbix.com/browse/ZBXNEXT-844) to allow Zabbix to better scale. It would be better if Zabbix supported a storage specially designed to store historical data like OpenTSDB.

Even better would be to abstract the server from the storage backend so it is easy to use and implement different storage backends. This would make it easy to use OpenTSDB store on a big site and MySQL on smaller proxied sites.