Signature Line

That's a good idea. The way it currently works the signature is not saved as part of the message, but applied when the message is displayed. So, if you change your signature the new signature is shown on all your old messages as well. It may be possible to change this, so the signature is loaded into the editor instead when you create a new message. Then you could make changes that would "stick" to individual messages. However, a change like that would leave all old messages with no signature, and this might not be everyone's preference. I can talk it over with the rest of the community team and see what they think.

Another option might be to use macros. If you go to Macros - My settings - PTC Community you can set a macro to use as a signature. 

Then when you start a new message you could add the signature and edit it if you wanted.

If you did use this approach you would want to clear out your actual signature field or you will end up with two signatures showing.