Is there a way to talk in global to everyone?
Or can there be a way to talk in global chat by typing something like a '!' symbol in front of your message?

Hmm i didnt think of that. i will try to find a solution. but i have no time atm. probably in 2-3 days

looks like this is already in the base game.
/reply /r <message> - Replies to the last player that whispered to you.
/shout /s <message> - Sends a message to all players including other forces.
/whisper /w <player> <message> - Sends a message to the specified player.
no command = chat in same team/force

is that working for you?

I can't test. got bug by new players.
Bug reported here: https://mods.factorio.com/mod/TeamCoop/discussion/5ac20016bc4701000bc18e92