First off I would like to say this pure speculation, I know no more than anyone else except for what is said in public, sorry no inside dope here. But what I can do is rampantly speculate at how I would construct it, then you can argue or implement a better idea--which of course is always welcome.
Guilds should be made up of a fixed number of people. 8 active players, and a max of up to 4 on the roster (yes I said assigned roster-more on that later). You need alternates because these matches at times are going to have funky times for some people, and eventually people are going to need a sub. The nice thing is as you have regulars subs can be the "non-committal types" that want to be in a guild just not full-time. What better person to have then?
Whatever the entry fee divided by 8 for your regulars (like a bowling league, when you are gone, you get a sub to take your place and quite often their fees are paid by the person who needs the sub). Say the entry fee is 100k dec, each member of the 8 will contribute 12.5k (or however each team's arrangements are made of course) and winnings also can work the same way whenever the guild wins something or receives some kind of prize whether it be daily or weekly, or whatever.
Now, each guild taking on each other will have their eight members take on each other in a battle against a member of the other guild. The 16 player tournament can be seeded or done at random based upon the contestants. Yes a single elimination tournament would take place in a short series of the first few, say best of 3 in the first two rounds, best of 5 in the final four, and best of 7 in the championship. Here would be the thing.... ultimately the bigger winnings would go to the top four places in a progressive style pot if there is a similar quantity or whatever the splinterlands team decides as prizes. That way if one guild destroys the other having all top 4 finishers then that guild would take home substantially more than the likely participation rewards the other guild would get--kind of like a wrestling meet, tennis meet, or a similar sports or regular competition based event like that.
Chatting with @davemccoy I think he hit the nail on the head having something specific as a reward that you can only collect as reward in guild play. Perhaps a new release of some reward cards, or something like that to spice up the idea of having many different levels of this thing becoming viable. Of course you want high end competition to be worthwhile but the underbelly, lower levels of guilds would be just as fun as well. Remember there are about 200 pro bowlers at the world at any given time, but there millions of amateurs that do it for fun or in their own level of competition as well. Building a retaining this underbelly and allowing the best of the best to have upward mobility would be a wonderful thing to make as an aspirational goal for the many playing this game.
You can always have say "blind" sidebets available as well. Say I am taking on player A from the opposing guild and there is a text box where each player can insert a number as a wager playing against the other player, making zero an option of course and having some kind of maximum, and the least common denominator by the two players would be the agreed upon wager, payable to the winner of the series.
I think all of this would be a great start. All I know is that DEC could likely be involved, and if it is to a heavy degree look out for those prices to go soaring on that. Especially with the rising number of players that is shrinking the pot for each of us as the game grows. Ah growing pains... I think eight to nine months ago we all would be ecstatic if the biggest problem right now is that the future crypto of the game is getting sliced thin and becoming too scarce because of too rapid growth of the game.
Enough about my thoughts, post your comments, questions, or arguments for or against anything mentioned here.