• Don't worry too much about finding existing characters unless things just fall into place or you really want to. If you do really want to, try using that character as a starting point. Otherwise, just try to think about what characters this precon might be a good opportunity for based on the colors and themes.
• Diversify- try and cover a range of different things, don't make your designs too close to each other. In my sea monster precon, I didn't want all the cards to just be sea monsters, so i made sure to throw some noncreature cards that work with the theme in other ways in there- I made a ramp card, a reanimator card, and a sea monster typal card that's a sorcery. Spices things up a little. Sea monsters is also a narrow theme, so I decided to make some cards reward big stuff rather than the sea monster batch, so that they'd work well with the theme but would also go well in other decks. I think it's important for these precons to have cards that work well in other decks, because they are ultimately still made for beginners and that lets beginners take apart the deck and get pieces for something else. It's less isolated. To that end, I like to make cards that work well with the deck theme but also hint at another deck theme you could do- the eldrazi in the sea monster precon, the board wipe in the saproling deck that would work just as well in decks for any of the other types mentioned, etc. Don't skimp on the cards that do help the main theme though- even with the narrower theme of the sea monster precon, I wanted to make sure to give ample reason to build that theme, so in addition to the commander I threw one sea monster batch reward into each color.
• Subthemes and backup commanders- this is an important one. Try and find one that ties in naturally with what the deck is already doing. Tokens for the saproling precon is perfect because a token reward is something you're gonna want in this deck anyway, it really doesn't stand out, but if you're building a deck for that backup commander instead of throwing it into this deck, that deck would look pretty different since it's not nearly as constrained. Same with the cheat-stuff-into-play theme from the sea monster precon. I knew I wanted the main commander to help you get sea monsters out, and at first I tried ramp but eventually I realized reanimator would tie in black a lot better and gives the deck more texture with a graveyard subtheme. From there, a backup commander that rewards cheating creatures into play felt perfect.
• Think about the theme's weaknesses- especially for the commander. In the saproling precon, I wanted to make sure to throw in wincons, so I threw an anthem of a sort on the commander, as well as the poison card, the board wipe to clear the path, Mycelium Networker to make your fungi all big. For the sea monster precon, its early game really hurts so I focused on that- the commander is on the cheaper side and helps you get expensive stuff out, I did everything I could to sneak smaller versions of the usually-big creature types in, and I threw in some ramp and such for the early game as well. On a similar note, think about what colors are more naturally represented in the theme and try to fill out the others. It gives more of a reason to want those colors in the deck.

Sculpted Photosynth 2/G 2/U Creature- Plant Mutant [common] (This card's mana value is four) Graft- Sunburst (This creature enters the battlefield with a +1/+1 counter on it for each color of mana spent to cast it. Whenever another creature enters the battlefield, you may move a +1/+1 counter from this creature onto it.) 0/0

Sarkhan of the Scalelords 4RG Legendary Planeswalker- Sarkhan [mythic] Cascade (When you cast this spell, exile cards from the top of your library until you exile a nonland card that costs less. You may cast it without paying its mana cost. Put the exiled cards on the bottom in a random order.) +1: Sarkhan becomes a 4/4 Dragon creature with flying and indestructible until end of turn. Creatures you control gain haste until end of turn. (He doesn’t lose loyalty while he’s not a planeswalker.) -2: Add RG. The next spell you cast this turn has cascade. -7: Sacrifice any number of other nonland permanents. Create that many 4/4 red Dragon creature tokens with flying. 4 loyalty

Captain Vraska 3B Legendary Creature- Gorgon Pirate [mythic] Menace, deathtouch If a creature an opponent controls would die, exile it instead. When you do, create a Treasure token. 1B, Sacrifice a Treasure: Create a 2/2 black Pirate creature token with menace. Activate only as a sorcery. 3/4

Piggyback Rider 1WW Creature- Human Pilot [rare] Each other creature you control is a Mount in addition to its other types and has saddle 2. (Tap any number of other untapped creatures you control with total power 2 or more: That Mount becomes saddled until end of turn. Saddle only as a sorcery.) Whenever a Mount you control attacks while saddled, it gains double strike until end of turn. 2/3

Ascendant Egotist 8/U Creature- Elemental [common] (8/U can be paid with any eight mana or with U. This card's mana value is 8.) Flying 1/1 "Once I was a simple mage. Now I am far more."

Coszcatl, Warherald 1RGW Legendary Creature- Cat Warrior [mythic] Haste Other Cats you control have haste. Whenever a Cat you control attacks, you may attach an Equipment you control to it. 4/4

Griselbrand Unbound 4BBBB Legendary Creature- Demon [mythic] Flying, lifelink Exile the top thirteen cards of your library: Search your library for a card, put that card into your hand, then shuffle. 8/8

The Maelstrom Spawn 1GUR Legendary Creature- Elemental [rare] Spells you cast with cascade have cascade. (Do it again.) Whenever you cast a spell from exile, put a +1/+1 counter on The Maelstrom Spawn. 2/2

Lanternlight Storyteller W Enchantment Creature- Fox Bard [rare] Lanternlight Storyteller gets +1/+1 for each lore counter among permanents you control. Whenever a Saga you control leaves the battlefield, put its lore counters on Lanternlight Storyteller. 1/1

Great Ally 4W Creature- Giant Warrior [rare] Team convoke (Your team’s creatures can help cast this spell. Each creature a player on your team taps while casting this spell pays for 1 or one mana of that creature’s color.) Great Ally enters the battlefield with a flying counter on it if a creature with flying convoked it. The same is true for first strike, double strike, deathtouch, hexproof, haste, indestructible, lifelink, menace, reach, trample, and vigilance. 4/4