In next RouterOS we will include small, but important change in firewall mangle. Every connection will have connection mark "no-mark" by default, and every packet will have packet mark "no-mark" by default.
This changes will greatly help to reduce complexity and efficiency of the mangle setups that require remarking of the connections and traffic. Traffic remarking will be avoidable.
For example with one rule it will be possible to ensure that this rule will mark only connections that was not marked before (connections with "no-mark" mark).
It will also allow to create queue for unmarked traffic, this way eliminating requirement that all traffic need to be marked in mangle to create proper QoS.