Alrighty, judging time. I'm going to be trying to look at these by how well they'd play within their given tribes, as well as whether or not they're an appropriate effect for their colours. Let's get into it!
Spoiler: Quiddle's Kari and Ragavan, Thieving DuoSo, off the bat, a fun idea to have a Pirate lord be Kari and Ragavan. 3/4 Menace for 4 is ok, nothing to write home about, so we'll need to look at the abilities. And... This is very strong. Your opponent is forced to choose between being fatesealed for 2 or ramping and giving an anthem boost. The trigger is an attack trigger rather than a damage one, too, so there's a very real threat of being pumped immediately. I'm not going to focus very heavily on limited, but this being only at Rare is insane for limited, I'd hate to be facing this on curve.
Looking at the abilities, Fatesealing has only ever been in blue, and only very sparingly at that. It's rightly recognised as an anti-fun mechanic as it can just outright lock your opponent out of the game. Tying it to an easily repeatable game action means that you can just keep on sculpting your opponents draws and prevent them from playing. The opponent can choose to not be fatesealed, but the alternative is to ramp and fix you and give a permanent pump to, at a minimum, Kari and Ragavan. The fact the treasure comes in untapped is also a big deal as it enables a lot of combat tricks/fast mana style uses. There are going to be times you swing Kari and Ragavan into certain death just for that attack trigger. This just honestly seems like an exceptionally strong creature and I don't like how few opportunities your opponent has to interact with the attack trigger, or the fact it opens up fatesealing to Red.
Spoiler: mystic1110's Underworld OverlordThis seems like a strong Assassin/Rogue payoff. Realistically, probably just a Rogue payoff. There aren't that many Assassins running around. 4 mana 2/3 Deathtouch Menace is a bit on the weak side. It's nice that you can take out both blockers, but you're probably not getting blocked, and 4 mana for a 2/3 deathtouch blocker means you really need to be blocking something huge to be making your mana back. So it's gonna come down to its abilities.
First, a quick templating note, the second ability should read 'Whenever a Creature or Planeswalker an opponent controls with a bounty counter on it is dealt damage, destroy it. When you do, that permanent's owner loses 2 life, you gain 2 life, and you draw a card." As it's worded at the moment, there's no provision for if the creature/walker is indestructible, and nothing happens if a Planeswalker dies with a bounty counter on it (you also don't have a way on the card to put a bounty counter on a Planeswalker, but there are enough Rogues with flash, and enough Planeswalkers that become creatures, that I can see it happening). The drain and draw effects are also just worded a little messily.
The first ability should also be slightly re-templated (and this is just me being a grammar nerd). You can use Harmonic Sliver as an example here. It should read 'Assassins and Rogues you control have "[Ability]"'
So on to the actual functioning of the card. I like this take on a tribal payoff. Bounty Counters have existed on the odd card here and there, and this is a nice way to continue the theme of them. Having any damage kill the creature/walker they're on might be a bit on the stronger side, but the bounty counters won't do anything if the Overlord isn't on the field, and we've had creatures that grant deathtouch to spells/ways to give deathtouch to boards before, and this isn't very far removed from that. Nice design!
Spoiler: ben-zayb's ArchaeoraptorSo I'm a big prehistory nerd.Which means you automatically win, congrats!Which means I'm always happy to see more dinosaur cards. This is, sadly, a bird payoff and not a dinosaur payoff, so I'll curb my enthusiasm a bit.
A 6 mana Serra Angel is pretty meh. A 6 mana Serra Angel with the potential to turn the rest of your board into Serra Angels is interesting. Having this as the top curve of a flying deck seems like it'd be pretty good. A deck full of cheaper birds becomes pretty fearsome when this drops. Beyond that, it's a very simple card. The buff will be most noticeable if you're using lots of smaller birds, much less so if you're using more expensive ones. Not much else to say!
Spoiler: Bucky's Squad FlankerFor a common, this guy's doing a lot. First, it's a little odd to see an Elf in mono-white. Not beyond the realm of possibility, Selesnya and Shadowmoor/Eventide both have mono-white elves, and I think Streets of New Capenna has a few as well, but still notable.
A french vanilla bear with upside is solid for a common. Having vigilance with the tap ability is nice, and I appreciate you specifying another Soldier needs to be involved before you can target something. Keeps it from effectively being a 3/2. The complexity may be a little on the high side for a Common, but I don't know if it's enough to warrant a bump to Uncommon. Having multiple of these is where it starts to make combat get messy, blocking and attacking become far more difficult if there are two or three of these down, and I think with that I'd probably say this should be Uncommon for the sake of Limited.
I do like this design, though. A nice simple early creature that rewards having a Soldier sub-theme.
Spoiler: Tom the Mime's Breathless OneThis is a tricky one to evaluate. Tapping/Untapping stuff is a nice effect to have for getting blockers through/maximising mana/denying mana. It may be a bit on the strong side given you can untap your own lands, and there's probably an infinite combo in that somewhere. With that in mind, it might be worth having it be either opponent's permanents, or non-land permanents. The P/T being equal to all Merfolk probably will mostly just mean equal to your Merfolk. They're not usually the splashiest of the tribes (pun not intended), unlike Elves/Zombies/Clerics/Wizards/Goblins, all of which regularly show up even outside of tribal decks.
I think Breathless One would be pretty well played within Merfolk decks, at worst you can effectively give your Merfolk a 1 mana discount (potentially more if using mana rocks/lands that add more than 1 mana), and that's not a bad floor to have.
Spoiler: Dr.Gunsforhands' Sisters of the SenateThis is a pretty interesting design. The colour identity is strange, I don't know that there's much that's green in it, but I can see an argument for it and it doesn't really matter from a mechanics perspective beyond how hard it is to cast (which, thanks to the hybrid mana, is actually pretty easy).
5 Mana for three flying 2/4's with deathtouch (and possibly some anthem buffs on top of that, though none of the three types are well-represented at present) is a good rate. It's difficult to attack past, it's a reasonable threat in the air, and if you can find a way to re-cast the Sisters (or even just cast a second copy), they start to get even stronger. I'd probably bump that up to Mythic for limited balance reasons, it'd be pretty miserable to run into someone running multiple copies in a draft, though I think it'd get first picked often enough that you'd have to have it as pick 1 or 2's to be able to do so.
Spoiler: Ok, but who won
Spoiler: Third PlaceIn third is Bucky's Squad Flanker. I do think it should probably be at Uncommon for complexity/Limited reasons, but leaving that aside the design is clean and can be used without investing entirely into the tribe. Simple designs work well for lower rarities!
Spoiler: Second PlaceIn second is Tom the Mime's Breathless One. It's hard to evaluate in a vacuum, but it seems like it has the potential to be a neat combo piece. The tapping/untapping is the main draw of the card, but it's nice that it also scales with the board. The main thing keeping it from the top spot is that it is just difficult for me to evaluate whether the lack of limitations on what can/can't be tapped is too strong or not, but I do like it.
Spoiler: First PlaceIn first is mystic's Underworld Overlord! I like the design a lot, the backwards compatability with existing Bounty Counters is nice, and having it for a tribe without too much support is also a nice thing to do. It also allows for a different direction to the mill theme for Rogues introduced in Zendikar Rising. Congratulations!