I think its called an X3...
Seriously though, there have been a couple of threads about trying to put together an extended base, but I don't think anyone has started work on one yet. It makes sense, and if anyone is going to design one this is the place that has the folks with the need, the knowledge, and the equipment to do it. I know people have put some thought into it, but no one has really developed it too far - at least that they have mentioned here in awhile.
The two main arguments against it I have seen were first that some people that looked into it thought the overall cost of doing it right would be prohibitive for its usefulness relative to just upgrading to an X3, and the second argument against making them had something to do with the fact cncbridges was a fraud and so by extension the entire concept is now inherently something bad or fraudulent or something. No, the second one really doesn't makes any sense to me either, and I don't know why that should keep people from now ever building another extended table similar to his, just that it seems to be a commonly enough mentioned argument against designing an extended table for an X2 that I thought I'd mention it. You can try to figure that second one out if you want.
The first argument by itself does make quite a bit of sense though. I personally am more of that opinion too, but I also know that there are a lot of people that would disagree and would be interested in just such a base.
There has to be cheaper alternative techniques out there that would make it economically feasible to create, the polymer concrete thread might be one place to start looking for a potential answer if you are interested.