When t5 was added, there were over 300 t4bc of varying strengths. I know because I was one of the last people to get there before t5 came out and I was ranked 330 or something like that.
There are less than 100 t5bc right now (this is a total guess based on where a friend was when she t5bc'd, and her VIP rankings) We will most likely not see T6 until there is a huge crowd at t5BC again, and even then, judging by the other ATA games, we will see more lands first.
We might also see Droid and Online access to PIMD, like KAW has, before we see any game play changes. This would possibly make the most sense because it would add revenue streams for ATA by adding new players to PIMD who would buy credits and DN.