Okay, I've read through most the responses here and here is my proposal. Some of this will probably be a bit unpopular, so bear with me, but for the good of everyone, I believe this makes the most sense.
First, ditch the extraneous rewards. No gems. No gold. The gems are a huge dividing point here. For nearly everyone, gems = troops = content. They aren't some extra premium currency, they are the main way you get things you both need to progress in the game and keep the game from becoming stale. Guild wars should not be reduced to a gem gambling station. The way it is shown in this preview, nobody stands to lose a significant amount of gems, and a very very very small amount of people will gain an amount that is, in the grand scheme of things, would be a lot for most people but end up being rather insignificant.. to them. Everyone else has potential to get a small bump, and some people will be sad to see that go, but really, it is holding the entire design of this mode back.
Now that extraneous rewards are out of the way, divide every guild into brackets. The higher brackets would still get better rewards for even competing in the bracket that they are in, but those in lower brackets would have a better chance at decent rewards.
So what is the ultimate reward, then? Well, still troops, but not quite. A new currency would be introduced - Guild Wars tokens. For a certain amount of tokens, you can purchase any past or current guild wars released troop. In keeping in line with how other troops are released, we could go as far as halving the cost of the most recently released troop, or restricting previous wars' troops from being purchased until four weeks after they were initially put into rotation. Since the rewards is troops and you have a choice of where to spend your tokens, you could go for mythic on a troop you like and mythic versions of these troops would be accessible to everyone.. albiet not every mythic at the same time, you'd have some choices to make.
So how does this generate revenue? Well, just sell the guild wars tokens. Simple as that. Cut out the middleman, for people that don't care about the pseudo competitive mode at all to still access the content (troops) that it brings to the table. Those that aren't competing (and winning) in higher brackets will never reach mythic on every troop unless they spend money. This may not be popular, but the way that is structured now, those that aren't in the top five probably won't ever be ascending these guild wars troops to mythic regardless. To keep in line with Guardians, these these purchases could be capped weekly so that nobody is outpacing the top dogs in guildwars by just dropping some money.
While we are at it, lets give the option for at least a few people that aren't going to participate within your guild to "opt out", which would of course affect which bracket you were placed in overall, versus guilds with the same amount of members fighting. Said opted out member would still get the rewards (everyone gets less for being in a lower bracket already), but we could have, for example, 25 vs 25 wars if we can't muster a full force.
As for sentinels, with gem rewards being taken out of the equation, lets halve the cost for leveling them across the board. If people still want the big stat advantage, it has to come out of their supply of gems, no recouping. Less people would be willing to do it, but every gem spent here is sunk from the economy. Those more willing to actually spend the 170 gems in the first place would still not likely to be getting them from the cash shop considering the overall cost, but given no chance at recouping them but maybe more troops, I personally might chip in to a couple of the stats if I could get my guild to at least participate in the wars (of course, I wouldn't force anybody into chipping in gems, still, but if I were Paragon that week and I thought it might help, I might do it).