What I've implemented so far would allow you to do a one-time multi stage mission that shows up on the geoscape. I haven't implemented triggering the events on a timer, allowing the event to reoccur, or timing the event out (synomium sites need all of those). None of those are exactly rocket science and I could fit them in over what I've done without rewriting anything.
I have no idea how those synomium and cruise ship missions triggered in tftd though - were they part of a normal alien terror mission involving a chain subs or did they "just appear" ? Certainly my experience was the latter.
The terrain stuff is interesting - if you check out the rules file in my build I've also externalised some of the terrain feature control into alien deployment (ie whether to show the craft, and 'guaranteed' terrain blocks like the alien brain, base control, exit lifts). In principle, you could do silly things from config like have a 5 stage alien base (this would need new terrain blocks for a lift to the next level)
No idea whether or not the terrain features code I put in is sufficient for the xcom2000 maps as I'm not familiar with them yet. I'm putting the guaranteed blocks anywhere on the map, so I couldn't do stuff like coastlines yet, where block position matters. But, I will take a look at the xcom2000 maps and see if they'd fit
