My guesses:
I am going to make my guesses based on the position of the "real" spain:
The civ will start on a large peninusula, possibly with a thin neck and mountains. On the other side will be the bulk of the civs.
There will be a "portugul" equivalent. This is another civ that shares the peninusula and competes directly for space.
I think doing the "arabs" would be interesting. There is a "rock of gibaltar (how do you spell it - Ugh!). Ths is a point that is close enough to the peninsula that you don't have to cross sea. A civ will be living there, and it will be challenging the player for space on his continent fairly early.
Most of the other civs will be on the main continent across the peninusula. This simulates france, germany, etc. There may also be other land masses similar to England.
The early challenge is to keep "portugul" small, and perhaps drive the "arab" incurrsion off the peninsula. Once these have been overcome, the player can deal with the other civs on the continent. For a conquest victory, you will also need to invade the "england' equivalent(s) and the "arab" continent. which means boats.
I am not sure about whether it will have a "New world" reachable only through ocean travel. This was certainly a part of spanish history, but might be "too much to do" and might require a large map which is slow.
The size of the game will be standard, but will have more than normal land if Europe only.