Overall, it sounds like a souped up version of the current GE idea, but placed onto maps. Which isn't necessarily a bad thing, as GE isn't overly interesting to many people as is. A few things I'd like to chime in about, in my opinions.
Part of the reason why there are way more people that play GE, versus GvG is a lack of accessibility. There are a ton of mobile players that play this game (likely a lot more than PC players), and they have no access to GvG to know what they are missing. There is also no incentive prizes to fight for in GvG versus GE, which surely influences activity. And out of the amount of PC players, many aren't high enough in era to participate in their guilds desired maps they want to participate in. Many guilds don't want to waste time on lower era maps that you mentioned are rarely used, because the prestige value is so low per tile it's not worth building up troops and burning on maps like that. You could consider a different approach to fixing this, by making the value per tile the same in every era across the board. If you did that, I will guarantee a lot of maps like Iron age, EMA, HMA etc will increase in activity exponentially. The reason why GvG fans want you to open up AF, OF, and VF maps is so we have something to spend those goods in our guild treasury on. If you opened up these extra maps, along with equaling the value on each map across the board, and integrate GvG into mobile app players, you will see the interest level of GvG rise a lot more than you think it will. Having a global/guild chat added to mobile so that mobile players can actually talk to their guild while doing GvG would also be a good idea so they are on the same page as the PC players on what they are supposed to be doing. Another problem of why lower maps aren't being used, is many don't hang around low maps forever because they want to advance in this game, and the low era goods don't accumulate as much in the treasury. This can also be addressed, if you integrate the ability to allow guild founders to convert guild treasury goods down eras at a 1:1 ratio (like 50 VF goods traded down to 50 EMA goods), to convert treasury to our needs. This would also eliminate guilds trying to convince players to hold back on advancing eras, just for treasury needs when they want to move up. This would also increase the amount of advancement on Arcs, Observatory, and Atomium (yes, in case people forgot about that one) levels from people above FE, because their goods will all of a sudden become useful again for treasury converting versus the now useless goods we are getting as is. Most don't bother even building an Atomium because they know it will be worthless after FE for goods into treasury, on current game functions. GvG experience can also be improved by making the popup screen when entering a tile to fight be randomized on popping up slightly left, or up, down etc to prevent several players of using methods of quick re-sieging/filling in 0.25 seconds as many have seen over the years on buttons they know will be in exact places every time (I know i'm not the only one that has seen this kind of thing with certain players that doesn't seem remotely possible for a human being to do consistently). I hope the improvements that you have in the works for GvG, will reduce the lag at recalc to run smoother versus 5-10 second delays in between attacks like we have seen recently...especially considering that only our low "5%" play GvG and all (just sayin). I do hope you reconsider the mobile GvG idea being shot down, because i personally think GvG is one of the most unique, strategic, and teamwork features you have in this game, that everyone should be able to enjoy. While I can understand coding is a "complicated" thing, the amount of revenue you make from your players I would hope you have a strong enough coding group that can figure it out.
The new battlegrounds does sound like an innovative idea that everyone can get involved in. It isn't a replacement of GvG as some are concerned about, although it does seem to be something new to try and draw people away from GvG, as it seemed GvG was being deemed somewhat obsolete on the original posting based on quoting current participation. I do agree with what one person said, that it may be a good idea to tie GvG, Battlegrounds, and GE together when it comes to guild ranking points displayed on your world. It would require participation on all features of the game to be considered the top guild on your world. It would show how complete of a guild you are, to be the best listed. Not just good in one aspect of the game which is currently just GvG, when it comes to prestige rankings. That would be up to game developers to find a balance of how to calculate all of these things into a reasonable comparison, to make such calculations. There is also the fact that maps reset and recalculate daily, while GE is weekly, and battlegrounds is 10 days...which would make a daily prestige calculation an interesting puzzle to figure out, if all 3 were linked together into it.