on second thought, I think I would kind of like this idea.
badspot said the development was "forget it" anyways
BUT, it would kind of make a split in the community, and chances are, more people would use v20 than v21.
This would have been a reasonable argument a year ago, but these days the community already seems to be very much split in two. I think this project would help reduce the tension between the two parts. If you like v21, play v21. If you like the old Blockland, you will have an alternative.
badspot doesn't like people using previous versions
Possible, but since Badspot seems to have lost interest in Blockland, maybe he will let this one slide. In the end, if the players are satisfied and the game survives he earns more money from sales.
What about the I.P? Like you said, a client side gui, but that lists all the v20 servers.... then you just enter the I.P address of the server, an play.
I am not sure what you mean. There can be a "Join server" button that simply makes you join the server.