Welcome, Katfish. If you are still satisfied with your current layout, maybe minimize it in your mind and think of your add-on as the major portion. In other words, if you have quite a bit of room to spare, now would be a good time to try designing something quite different in terms of variety, operational and functional capacity. You don't necessarily have to change the scenery and setting theme if you don't want to, but it may actually be a good idea to have two separate themes with a divider or something, even a large hill, between them. You could have an interchange between an eastern and western railroad if that appeals to you and opens up new design and playing opportunities for you.
Just be sure that it works where it counts...where the tiny trains actually have to move across to the new part without a proverbial hitch. Maybe your current layout has grades? Make the new part flatter like the prairies, or else have a useful/functional yard in the new section so you can change from watching trains run around the loop on the first part and actually switch to make up new trains. It's a thought. I added a yard on my current layout after not having one on my first, and it has added a whole new dimension to my train sessions.
You should get lots of excellent ideas from the other responders. Good luck.