Hi, welcome aboard.
Some answers:
1.) Yes, it's cross-platform. It's being developed natively on Linux
in C++. The previous version was in C and was being developed on
Windows but is still cross-platform.
2.) There are currently no plans for extensibility. It could be a
possibility in the in the distant future. More on this later.
3.) You could do that, but you'd have to extract the civ tilesets in
the same format U1 uses. It's not something I'm going to venture into.
Now a bit about the project... the project is really about learning
how U1 works. It's not about developing a U1 equivalent of Exult
Studio or anything of the sort. It's unlikely that it ever reaches
remake status. In fact it's currently on hold since I'm busy with
University. You can consider it more of a set of tools to look at the
innards of Ultima 1 and learn how a real game works, or at least how
it did back in the days before I was born.
A lot of Ultima remakes have been seen promising a lot of stuff and
then dying because they underestimated the task of creating a game.
This project never really had any plans; wherever it gets is always
better than nothing.
If anyone wants to make it extensible, or make it work in a
multiplayer world, etc, it's up to them. The project is open-source
and they can do what they want with it. What I myself do with the
project is purely dependent on the time I have available (which right
now is zero).
If you want the source, let me know and I'll send you the latest
version.
Daniel