While I love the minimal friends system in ToS, this is one small change that's been on my mind (and probably other ToS players' minds too) for quite a while.
There should be a way to message friends while they're offline so they can see the message when they get back. As of now the only way to contact your friends on ToS is if they're online at the same time as you. There are quite a few friends I've made on here who aren't active anymore, and as of now there's like no way to contact them via ToS unless they're online.