Well, relevancy's a weird thing. I mean, clearly Final Fantasy is still relevant, people still talk about it, it still sells well, it's not irrelevant.
I don't think XIII has the greatest story ever told, but then, none of the Final Fantasy's did. Seriously, go back and play one, any one, you think that shit has fantastic writing? Isn't silly as shit? So do I think the storytelling of XII or XIII pales in comparison to VI or something? Not really. So I don't think it's the quality of writing that's declined here. But hey, I'm always up for better writing.
Do I think the character designs have faltered? Nope. They've always been silly.
Music? Eh, this will differ depending on who you ask but I still think the series has good music.
Gameplay? Well, excluding XIII and XIII-2, and also VIII but I try and forget that game, I think the gameplay had been consistently getting better.
So what do I think has contributed to Final Fantasy, not losing relevance but maybe, losing it's prestige, shine or luster? Graphics. Storytelling. Time. Not of the Final Fantasy series in particular but in everything else. No longer does Final Fantasy stand above a sea of janky weird ass JRPGs with it's relative non-jank but also weird characters. No longer does it stand leaps and bounds graphically superior to other RPGs when they all look "good enough." No longer is the character design something to our imagination or relegated to those who view the instruction manual but it's on our big screens in all their pixelated glory for us to ridicule. No longer is the RPG the genre where you can do more than any other game when you have games with destructible environments, swimming mechanics, cover mechanics, stealth mechanics and the like, now the RPG feels like the most restricted gameplay format around. No longer is the RPG the genre that tells a story when almost every genre can tell just as strong, weird and flimsy a story.
I do think that also, us gamers growing up does have something to do with it but I don't think that's the sole issue. To me the RPG by it's very definition is stuck. It really can't evolve much. You can play with the formula a little but too much and all of a sudden "it's not a RPG," where as you can get away with throwing in RPG elements into a first person shooter or whatever, come up with something with better gameplay, equal storytelling capability and the like and you don't get the same hate from your fan base. Therefore I don't think the JRPG ever can become big again because I don't think it's them that have declined in quality that's the issue, rather than the consumer now has more choices for storytelling in games and the average consumer isn't so strict on what genre their game is and where it borrows elements from. The JRPG fan, however, is a lot more demanding and strict for what they consider a JRPG to be and thus won't tolerate too much creep from other genres into their games making the genre somewhat stagnant and tired.
So the problem is the JRPG fans themselves I think. They want the same tired mechanics of years past, the same fantastical character designs from the past, the same batshit insane stories from the past and when they get it, they shit on it. And 9 out of 10 times they're right to shit on it.
I'm of the opinion that all genres deserve to exist. There's probably people that lament the verticleness of modern shooters and just want to play Doom 26 without having to look up or down, I think those people should get their game. I think people wanting a new Final Fantasy VI-2 should get their game. Thing is, I think gaming's gotten large enough with enough genres offered now that some publishers need to start to learn who their audience is and not worry about mainstream success. There will never be a Final Fantasy more "relevant" than FF VII because that's all been done before now. I don't care how fantastic looking you make it it will never again conjure up that "holy shit this is next gen shit here" emotion from anyone. Everything looks good enough now. The rise of the JRPG happened because not of anything the JRPG did right but because it did things that at the time no other genre could do which coincided with a time where technology advanced rapidly which allowed an established developer like Square to stand at the top of what was possible and because since so many of our games at the time on consoles were Japanese and the RPG was a popular genre there we got a lot of them, of which Final Fantasy stood at the top. Recreating that market is the only way to make Final Fantasy "relevant" again and frankly I think that's a horrendous idea.
So, to sum up, forget making it "relevant" and shooting for that same level of appeal that VII had and just make the game you want on a reasonable budget and call it a day.
Also, put Lightning in everything, keep Ali Hillis forever and give her her old breasts back.