BTW, First Post.

I've been playing a bit on the CS to make an island and I just wondered... has there been any attempt to catalog online the exterior cells used by modders as a means to minimize the chances of mods using the same cells? I just wanna check if where I'm laying the groundwork won't interfere or overlap with any other mod online that I'd be interested to try as well...

I don't think there is anything out there that does a complete catalog of all changes, but here are a few that I found;

http://www.mwmythicmods.com/MAPoMODSbyCyronaut.jpg

http://webzoom.freewebs.com/vagara/In-game%20Map%20-%20Morrowind.jpg

As you can see, there is very little real estate left.

Thanks!
It appears like I got lucky, the spot I'm tinkering with looks like is still free...

Unfortunately for you, maps that Miles_Acraeus provided are horribly outdated and irrelevant (for one, whole east and south side of TR is not present, while silly stuff such as heaven and hell is). Unfortunately for you and all the modders out there as well, there is no updated map because there are way too many landmass mods out there both made and being planned.

That being said, just make you mod wherever the heck you want (while trying to avoid at least TR incompatibility) and don't worry too much about it.