Since 1.7 you can not visit a lot of the worlds you owned due to the credit system so what I a mess proposing is an owner only mode or spectating mode.
You would be able to build on the world but no one else would be allowed on except for you.
You can build on it but it may not save unless you have 100 time crystals in hand.
It will not credit it but it will save it to the backup of your world until you add credit again.
Again, only the owner of the world can join the out of credit world.