Well the thing with +G +C and +R is they are not available on all servers, and they don't always do the same thing on all servers. For example, some servers use +C as "no colors" others use it as "no CTCPs", but nonetheless I would support adding new modes, just in a different way. Something like a "channel central" section of alt+o where you can define additional modes present on networks, like
Network: DALnet
[Add Mode]
Clicking add mode prompts you to enter the mode letter, say R, and then a description "only registered nicks may join", a checkbox of whether the mode accepts a parameter. For the big networks, mIRC could have a builtin method of storing their additional modes so that for example for DALnet you wouldn't have to add +R, because mIRC would know to do it on its own.