Good points and questions all, @yabapmatt.
Since you're close to the source (and keep in mind I am NOT a gamer), let me start my answer with a question: "What does Splinterlands offer that people can't just get from MTG?"
Not saying that as a critique, but as an opening to a deeper look at what successful niche markets can do, and how they provide value...
I was an early adopter of eBay, and it was the coolest thing ever! In the late 1990's, it was essentially a giant online fleamarket, collectibles trading place and a venue where people went to look for stuff for their 1800's model soldier collections, or period doorknobs to restore a Victorian house, or weird handmade art stuff. If eBay were still that, we wouldn't even be having this conversation.
However, eBay is NOT that... what seems to always happen, happened: eBay became successful, made a lot of money, became a public company and switched their focus from "making USERS happy," to "making INVESTORS happy." You make investors happy by maximizing ROI, and you maximize ROI by finding as much low hanging fruit as you can, and focusing on that. And just like Wal-Mart supplanted the hometown grocery store, large corporate merchants supplanted individual sellers.
OF COURSE people want cheap cat litter and diapers... what gets "lost" in that mix is a way to buy Aunt Bertha's Hot and Spicy Pickled Okra.
Ironically, eBay is actually a decentralized marketplace... eBay owns nothing, they simply facilitate commerce between buyers and sellers.
Craigslist is a result of an area in which eBay was weak — local trading — and became weaker as a result of policies to make contact between buyer and seller less and less and less... for fear that someone would "trade around" and thus avoid paying fees.
I'm one of a shrinking number of individual traders still using eBay... but it's very frustrating to have your wares on a venue where you're often giving up 15% seller/transaction in fees... only to be a disadvantaged citizen who doesn't get the discounts and volume benefits of sellers like Sam's Club, Target, Office Depot and such.
Options? Well, you can set up your own web site. But that's a lot of work and marketing becomes a 24/7 proposition. Think of it as the difference between setting up a roadside table for your home grown veggies and having a booth at the Farmer's Market. The other option is niche markets.
Getting back to your original question of Hive and adding value, a working example is a marketplace called Poshmark (started in 2011)... grew out of successful handmade and vintage clothing sellers on "old" eBay losing ground to mega stores and cheap Asian imports. The important point I wanted to make is that my wife is a high level trader there... and one of the "features" is that the money from anything you sell stays ON the site until you specifically request it be sent to your bank account. The value in that — as my wife well knows! — is that she may make $200 off sales, but she'll end up spending $100 with other Poshmark merchants before withdrawing any funds... in other words, there's support for an inner economy to develop.
Banking the unbanked, I'd have to agree remains more of an ideal than a reality. But along with what I am talking about here, you have to offer people a reason to become "banked." Saying something like "Here, you can use Hive ANYWHERE!" doesn't amount to a hill of beans if you can't DO anything with that Hive. Now, if you're sitting somewhere in the bush and make really cool jewelry from fossilized shells and realize you can trade that to someone in Germany for a virtual currency that allows you to buy three chickens frpm someone three villages over... then it starts to make more sense.
Reaching critical mass, that would depend many factors including what we would decide "successful" looks like. If you look purely at marketplace logistics, from 20-odd years of watching this gig, I'd say you need a minimum of maybe 200-300 "serious" sellers and 1000-2000 "casual" sellers to offer a marketplace with sufficient variety to be "interesting." Some of them would also be buyers, of course... and you'd need a minimum of 10,000-ish potential browser/buyers in order for sales to start gaining momentum, which would be a 3-5 year proposition, my guess.
Well, I'd better stop but at least it touches on some of your points... without doubt, the entire thing would be a very complex process... but a bit of a "template/example" already exists in the Splinterlands marketplace and with the LeoShop.
Just WOW.
Other value 'proposition' for hive; as a learning tool.
Because with comments/posts like this one, I learn more in 30 min that I would learn in a full week of university.
Thanks for this.