So far, we've seen three of the seven: Mercurius, Mars, and Sol (pictured above, respectively). The main theme of the Seven Luminaries is the use of the Mana keyword and Mana counters, which can be used to produce extra will to play Ancient Magic spells. The Luminaries each have a familiar resonator, which can be removed from the game from your hand as you Judgment to give them more Mana counters. With Mercurius and Mars, they each have a signature spell that you'll be spending most/all of your Mana counters to play. While we haven't seen all of Sol's cards, he seems to just be a more flexible Mars. The only difference each Ruler has is the color of will their Mana counters can make, and the benefit they get from removing their familiar. Mercurius gets Flying, Mars gets First Strike, and Sol gets a stat boost.
That being said, each deck plays very differently. Mercurius is a build-up, using ponder and search cards to grab certain combo pieces before flipping for a huge tempo swing with her Ancient Magic. Mars is a rush deck, aiming to get the opponent to 2000 life as fast as possible so you can flip and burn them for the remaining 2000 with his Ancient Magic. Sol seems to encourage less of a"huge turn" and more of spending and gaining Mana counters a lot over the course of the game, and playing higher-cost spells for cheap with them. However, these different play styles seem to be thanks to the Ancient Magic spells, rather than due to the traits of the individual rulers. The spells do a good job of representing their colors, but the Rulers themselves are only as good as the tools they're given, and Force of Will seems to be pretty hesitant about giving them too many tools. The idea behind making different J/Rulers is to encourage various play styles and decks, yet here we are with 3 near-identical Rulers whose only differences are artificial. The only reason Mars and Mercurius play differently is because the game basically says "You can only play this one card if you're playing Mercurius." So, instead, I propose this:
First off, shout out to the guys on the reddit who made the card editor app. You can find it here if you want to check it out for yourself. Here is a redesigned Mercurius, keeping everything about her card the same except for the attribute, which I removed. Imagine, rather than the 3 Luminaries we currently have (and however more we're going to get), Mercurius was the only one. She produces void will with her mana counters, rather than water or any other specific attribute, and we keep our current pool of Ancient Magic cards. You could splash any color into her deck and have a working skeleton. Want to use Rising from the Depths? Make a water deck, throw some strong water resonators in there. You'd still need magic stones to make the water will to play the blue Ancient Magic cards, but all the void costs would be paid with your mana counters (and some of the spells would be re-costed in accordance with this). You could go Red/Blue to play two different kinds of Ancient Magic, or Red/Green, depending on what a Green Ancient Magic would do. Any combination of colors would be available to you. The amount of deck variants we'd see with Mercurius would be amazing, and she'd only require the absolute minimum amount of support each set to feel refreshed.
That would free up two Ruler slots (Mars and Sol), plus a potential 4 more from not printing the other Seven Luminaries, that could go to other Rulers that would create different play styles or deck variants. Since Ancient Magic is just an identifier on a card, they could be printed for the rest of the cluster, meant to be splashed into decks of their color, but still usable by Mercurius variants. A card like this has the potential to be so flexible without actually be broken. Rulers that don't need Ancient Magics wouldn't have to compete with her for a spot in your deck.
The way the Luminaries are designed now, though, just doesn't sit well with me. They clearly are designed to work towards their respective Ancient Magics, but that just feels like a crappier, less consistent version of a God's Art. You have Rulers that basically say "This card let's you play X card, if you removed Y card when you performed judgment." What if Mercurius instead read:
Here, the Rising from the Depths effect is built-in to the card, and the same would be for Mars. This removes the inconsistency of needing 2 additional combo pieces (The familiar and the Ancient Magic) just to play a card. In exchange for losing the Mana counter mechanic, these new Luminary cards could have built-in cost reduction on this God's Art. For example, maybe Mercurius would let you pay 1 less for each card in your hand, keeping in-theme with water's drawing mechanics. Or maybe the cost is decreased by you having more cards in your hand than the opponent does. There are plenty of ways to spin this while removing all the unnecessary combo pieces the cards currently have.
The argument here is that "These are powerful effects, they're meant to need combo pieces and be a little inconsistent to compensate for how impactful they are," which is a fair argument. However, that reminds me of Exodia from Yu-Gi-Oh, which is an insanely powerful effect (Instant-win), but very inconsistent. Obviously the combo's here aren't on that scale, but it's the same idea. You're looking at a deck that will properly go off every few games, and probably get steam rolled every other game. Now, Mercurius has seen some competitive success, but that's really only thanks to Captain Hook putting the opponent behind enough to win from sheer momentum. Mars hasn't seen nearly as much play, and for good reason. From a card design standpoint, a hit-or-miss deck isn't very well designed and will put people off from playing it. I respect the idea of a deck based around making one huge tempo play, but I feel that it could have been executed much better. I'm really not looking forward to seeing four more of these cards, either.