Regarding GvG:
If GvG were available on Mobile it would be far more popular. Even if not the best experience, players would attempt even if only to help their guild.
There are plenty of players that play on puffin. I understand that it's not bad on tablet, except a bit slow at times. It's hard on the phone, but there are plenty of players that are able to fill DAs and even fight in small scale. Definitely not optimal for if having to lead fighting across several ages.
GvG is pretty complex. There could/should be either a step-by-step tutorial or a video training. If there was a practice area where a player could learn the mechanics without serious points/unit loss, it would definitely bring more into the fold.
One of the bigger challenges for GvG is that so much is decided at recalc. Because people's schedules, it's really hard for a lot of people to make that specific time.
AF, OF, VF = no man's land soon. Before SA:M, players in FE were still able to play in AA. Once SA:M is out, those units are strong enough that they will more-or-less make all other units obsolete. That makes the jump from FE to AA a 5-6 month wait. That will definitely discourage a lot of players that want to GvG but don't want to be in no-mans-land where their age-specific arc and units are both useless for GvG. It might not add more players to the pool, but if definitely could shrink the pool of players.
There's a lot of players that only play FoE for GvG at this point, and many of them are pretty big spenders. They might buy into the BG concept, or they might just get POed and quit.
Regarding Battlegrounds:
The general concept is good, but there are definitely a few major drawbacks
It's not a real-time and fast-paced as GvG and will lack the "skill" for sieging, breaking sieges, slipping DAs, etc.
It's one more thing to be doing. Managing Daily, Events, GE, GvG, Settlement, SA Colony, and soon to be BGs... It will become too much for a lot of players. No idea how to balance that.
And BG questions:
How are groups matched? Guild size? Guild Rank?
Is there a global scoreboard?
How will BG's affect guild power/ranking within a world?
How will BG's affect guild level?
Are the fights prearranged like GE (so you can find the best matchup), or random like GvG?
Will the fights always be 1 wave? 2 wave? more than 2?
Are all ages weighted equally?
When guilds need to attack in BGs and defend in GvG how will that work? (obviously, they'll have to choose. Could lead to some pretty frustrated folks -> non GvG guilds could focus their attacks around recalc and make it harder for GvG focused guilds to take proper countermeasures... lots of unhappy campers to follow)
BG ideas:
Could this be setup more like an area ladder rather than GvG? Fighters can team up and fight other fighters paired up. 2v2, 3v3, 5v5, 10v10, etc.
Teams/guilds could be paired up with other teams real time instead of having to watch maps endlessly.
Anything that's more real-time and requires less endless map watching is better.