You've gotten a lot of advice so far, but two points really jump out at me that need to be emphasized. First of all, the only rules that apply to your layout are physical ones, such as how many locos can you fun on the DCC system you chose, how much resistance is in your track, how tight you can make your curves and still get reliable running, how smooth your track work is, etc. There is absolutely no requirement that your layout resemble a real place, or even have any scenery at all. As long as your trains have room to pass on adjacent track, there is no reason why you can't cram track into every available inch of layout space. You do what you want. Consider, though, that most people aren't content to watch a train chase its tail all day. Eventually, they want to actually DO something with the trains. This is where structures, scenery, and other non-track items become important. It's much better to think this stuff through in advance than to rip out and redo large sections of a layout.
The other point is that, so far, your thinking is squarely in the box. You have all this space available to you, and you're thinking of a small rectangle in the middle and a ton of wasted space all around. Think about how you can use that space (the layout design forum will help a lot, especially the two sticky threads with layout plans). 10 x 15 is a very nice space, really; it would be a shame to waste it on a 4x8 or 5x9. If you really CAN relocate to the other part of the basement, don't think about moving the whole layout; think about how you can selectively use some of that space to complement your layout, like a yard, a return loop, or even a whole separate section.
Since you're new to the hobby, be careful of biting off more than you can chew. Start small, but plan to build modularly, so that expansion is easy, and planned from the outset.
