Goal 1: Make a Standard deck for under $20 (mid).
Goal 2: Use Swarm of Bloodflies, a card that I thought was cool from the first time I saw it, but at base 2/2 is just difficult to justify over stronger 5-drops.
Stipulation: Tarkir Block only, so that I can add to it but will not have to remove anything for a while.
This deck uses synergies between various cards that affect +1/+1 counters. I looked at B/G and B/W, and decided that full Abzan has the best combination of abilities. It should be fun to play once it gets going, but the complexity level is fairly high, so I recommend a solid understanding of stack and timing.
Feel free to comment. I have not played it or opimized the mana; it's all an educated guess.
This deck relies on the synergies arising from the two rare enchantments, Hardened Scales and Abzan Ascendency. Without them, many of the creatures are fairly ordinary, but with them, especially in multiples, you can turn even trades or losses into profit.
Example of a good open:
T1: Tapped land.
T2: Hardened Scales, Servant of the Scale. Servant comes in with 2 counters at 2/2.
T3: Kheru Bloodsucker.
T4: Abzan Ascendency (and a tapped land or open mana). Add 2 counters to each creature. Servant is 4/4 with 4 counters. Kheru is 4/4 with 2 counters.
T5: Now you have options. You can play something new, or you can use the Kheru's sacrifice ability at the optimal time, if appropriate. If Servant is sacrificed to Kheru's second ability, Kheru's drain ability, Ascendancy's token ability, and Servant's death ability all go onto the stack.
Net result of sacrifice: Add 5 counters to Kheru from Servant's death, add 2 counters from Kheru's sacrifice ability. Kheru is now 11/11 with 9 counters total. A 1/1 flying spirit token comes into play. You gain 2 life and opponent loses 2 life. That's a lot of effects out of 4 cards.
Other synergies:
Bloodflies in play, cast Duneblast. Bloodflies gets 1 counter per creature killed, plus 1 more per creature killed per Hardened Scales in play. You also get 1 spirit token for each of your non-token creatures that died for each Abzan Ascendancy (not that you're likely to need the tokens if the Bloodflies are big enough).
Daghatar and Quartermasters both can move counters -- but they add a bonus counter per Hardened Scales. Daghater is particularly good because he can move counters from other creatures -- even your opponent's -- and increase them at the same time. In multi-creature combat, this can be devastating, and your opponent may forget or mis-count exactly what you are able to accomplish by moving counters.
Gleam of Authority. I feel like this card is undervalued, but it is an Aura, so it's not surprising. Add it to effects like Daghatar and Quartermaster, and it can really complicate combat. Remember that the creature with Gleam gets to attack untapped, and can tap later in combat (or on defense) to use the bolster ability. Other counter-moving/creating abilities can change which creature the bolster hits, Hardened Scales can multiply bolster counters, and the creature with Gleam gets bigger every time one of your other creatures gets more counters.
Ainok Guide is fairly boring, but it can come in with multiple counters if Hardened Scales is in play or help with land problems. I was torn between it and various creatures with outlast or megamorph, and I went with the Ainok for speed. I also tried to keep the early-game white to a minimum so it could be a splash in the mana supply. For a few dollars more, you could add some stronger 2-drops like Anafenza Kin-Tree Spirit and Avatar of the Resolute. One definite challenge with this build is the lack of exciting low-budget creatures for 1 - 2 mana other than Servant of the Scale.
There are a lot of cool Abzan cards that play tricks with counters, but I tried to keep the overall mana curve low because you will want to use the mana-activated abilities on some of these creatures as well. I also tried to maximize the number of creatures that come into play with counters so that you don't have to waste turns, mana, or other cards to benefit from a Hardened Scales in play.
Abzan Advantage or Feat of Resistance might be good enough to main-deck ahead of something like Inspiring Call. Both can be used just to add counters even if you don't need their other ability. Inspiring Call at best can be card advantage and surprise protection from most kinds of removal, but if you don't have a creature with counters in play, it does nothing.
Retribution of the Ancients is an insanely cheap (and reusable) way to destroy (or threaten) creatures, but it also falters if you're playing from behind. You also don't benefit from multiple copies in play. Very nice for things like Master of Waves or various hard-hitting weenies.