It would be great to have an action for unavailable proxies to switch hosts monitored by these proxies to other proxies.

For example:
1. ProxyA is not available;
2. Zabbix knows that fact;
3. Execute some action to choose another proxy for hosts monitored by proxyA
3.1. Distribute these hosts between multiple proxies (for example, using dynamic calculated max nvps per proxy)
3.2. Switch all hosts from ProxyA to one different proxy
4. When ProxyA became available, choose behaviour:
4.1. Switch hosts back
4.2. Using sticky policy - leave hosts on current proxy
5. Groups for proxies, like, it is possible to choose "backup" proxy only from dedicated group.
6. Weight of proxies to choose the best one, for example, with minimal load.

In this case we can say the proxy after it became available, your hosts currently under monitoring by different proxy(ies) - remove your historical data if you have something to send.

The feature could be very useful in Cloud environments along with proxies auto-registration.