Cross-world play for Battlegrounds is something we have considered in the beginning of the development.
In fact, some of our developers wanted to have this so strongly that they went the extra-mile to investigate its feasibility.
However, as it has been pointed out already, it's not that simple and it comes with a lot of potential edge-cases and risks (i.e. what happens if one game world goes offline accidentally?).
I wouldn't say it's comepletely off the table, but it won't happen in the near future, that's for sure.
Really happy the devs have given crossworld play a lot of thought. I had two ideas for cross-world GBG related play that would also take into account the fears expressed by some players that their less-developed guilds would get smashed by higher ones.
1) Shorten the current GBG season by two days. In those two days, at the end of the season, invite the top finisher from each island (or the top two finishers, etc) to compete in a cross-world island whose outcomes would NOT affect MMR but there would be more rewards, more resets of attrition at random intervals, and perhaps an AI guild. The AI guild could have a max number of players equal to the average of the players of all the invited guilds on that island. With randomly generated AI players who mirror random members of the invited guilds. And the AI guild's participation after the first hour would be no higher than the average participation of all the invited guilds in the previous hour.
2) Instead of inviting the top 1 (or 2, etc) guilds from each island to the two-day post-season play, invite the top 1 to 10 individual players from the whole island, regardless of guild and place between 50 and 100 of them at a time on a cross-world GBG-style island (players who manage to qualify on multiple worlds would find themselves on multiple islands only to avoid self-help). Plenty of rewards and resources to be fought for and won during those two days. Again, I would suggest multiple resets at random times. Instead of victory points, you could accrue the rewards based on which sectors you held. So some sectors grant fps per hour. Some units, others goods, etc.
The second idea acts as a special incentive to be a top player on any island. On the other hand, it is Guild battlegrounds, and the first idea allows a guild to go crossworld. I also like the AI guild. But honestly you could have an AI guild roaming the island in the second idea, too.
The concern over world outages, etc, is not eliminated here. But keeping crossworld play to two days reduces the risk considerably, and as I would make all cross-world play MMR neutral, no guild would be penalized even if they just decided not to participate.