So, it turns out that while I was busy thinking of how I might make my own modular terrain format, and hoping that I wouldn't have to reinvent the wheel, a thread on the cardboard-warriors forum sparked a conversation on the exact same topic.
I really, really like TLX. Some people seem to loathe it. I'm trying to identify the things people don't like and see whether it can be improved on. Here are some pictures of a simple idea - remove the anchors that many people hate and replace them with, well, nothing:
Hopefully you can see in this picture that there are holes on the tile where there would normally be anchors. This would allow posts to slot right in there.
The idea is, if you aren't using a hole, you'll have a little cover for it. My *hope* is that making the cover will be easier and less noticeable than the TLX anchors! If I fail on either point, then this is all a waste.
Another challenge will be figuring out how well other vendor modular terrain systems will fit with this...