You are correct, at this time being able to save an email campaign as a template isn't currently available in our new Third Generation Editor. This is a feature request we have submitted in your account to the appropriate teams. However as a workaround you can make changes to your template and continue to copy this template for future emails. You can also put your template into a folder for easy access!
If you can do any coding at all, then you can save your custom template a different way. This is what I do.
First, take your time and build the perfect template with everything you like.
Go into Advanced Editor, Select All, and Copy.
Open a plain text document in Word Pad or Text Edit or whatever program you use for plain text. (Don't use Word or any similar program. It will trash the white spaces and the code, even in plain text!)
Paste the code into it, and save it as plain text. Make sure it has a .txt ending, or you can use .html if you edit your code by hand.
The next time you want to create a newsletter, start the first screen of a new email, open Advanced Editor, copy the code from the plain text doc and paste it into the Advanced Editor screen. Save and you will see your custom template.
This is the cleanest way to keep a perfectly clean copy of your template. If you have any coding skills at all, you can go through and clean it up by hand, getting rid of any excess code. If not, just use it the way it is.
If all of this sounds too hard, then another way is to create one newsletter that you like, then the next time, to make a copy of it to start your next newsletter. Each time you recreate, it's best to go back to that original good newsletter so it stays cleaner. If you make copies of copies of copies over time the code will get worse and worse and you'll start to have problems.