Allow a function to allow server owners to add a friends list, this way, you could use a command such as /addfriend [Name of player], then to view your friends you can use /Friendslist, perhaps to progress the idea, you can use /fc [message] an abbreviation command for "Friends Chat", to talk to everyone on your friends list only.
When you add someone as a friend, their blip on the map should turn into a different colour, but if they're on the opposite team, you would be unable to see their blip for stealth reasons.
This means that your friend list wouldn't be global to ace of spades, but only to those servers who have the function enabled, meaning your friends list for every server which has this function enabled may be different, thus when your friend comes online on the server where you've got a friends list, you see them in a certain colour if they join your team.
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Second idea is that currently we need to use /PM [Full name] [Message] to send personal messages in game, but I think this should change:
/PM [ID] [Message]
The ID would be the same number infront of the players name on the TAB list.