So Feln Midrange is one of those decks that's kinda floated around forever, and this is my take on, post- into shadow. The big additions to this list from decks that have been floating around since set 1-2 days mostly come from the new expac.
Vara - Vara is very good against aggro, and a pretty average card in midrange mirrors, and a beefy threat against control. Yes there are non-synergistic situations with champion, but basically, make them kill the vara, and if they can't - play champ, if they won't - kill them with vara. In controlling matchups it can be pretty important to put pressure on their removal, so try to force them to use removal spells on cards like ripknife and vara before you play out champion for maximum effect.
Shakedown - this card is nuts. Seriously. It's so good so often, and can frequently disrupt your opponent while giving you additional draws towards power. I can't say too much about how to play shakedown, it's kind of something you have to feel out for yourself.
Last Chance - I think this card is better than dark return, and my theory is based upon the fact that - when you have one more power than you need, dark return plays the creature with +1/+1. when you have one less power, dark return does nothing at all, and that kind of tempo loss can be absolutely backbreaking, especially when you're working with limited mana.
Some of the other card choices i've had questions on, just briefly -
Midnight Gale - This card puts on tons of pressure, and has the ability to gain life in aggro matchups. 4 attack is a lot of attack, and evasion is a big game, especially against decks that don't have torch. Sometimes this will get torched and that feels bad, but getting it vanquished in this deck actually feels great, because basically every other creature in the deck is a vanquish target as well. (No, giving this card 3 attack would not be a buff, stop.)
Champion of Cunning - This card is still good. Doesn't matter that I'm playing it in a deck that cancels it's aegis occasionally, or that it isn't the most consistently activated thing on turn 5. CoCu is not a bad card, even when only one side is turned on, so long as you think before you cast it.
Feeding Time/ICB Split - So. You may be wondering why this deck plays 2 feeding time and 2 icb. and i'm here to say, I don't really know, but both are good in different places. If my matchups ever start feeling like one is strictly better than the other, this split may change, but for now, this is how it goes.
This deck runs primal market because Jennev merchant is a really good threat. If shadow merchant had a respectable body, I'd probably run shadow merchant instead, but shadow merchant sucks, so here we are. Don't be afraid to play your jennev merchants on curve, just be aware of what you need in the matchup.
Permafrost - Jennev Merchant for permafrost can frequently be a backbreaking tempo play. Permafrost has some minor anti-synergy with vara, so try to sequence it if you have a vara in hand, but permafrost as hard removal for 1 mana is frequently too good to pass up.
Unseal - If permafrost is the card that is almost always a good market target, unseal is the opposite. Be very very careful before you market into unseal, because it can frequently rot in hand if you aren't putting on enough pressure. I almost exclusively market into this card when i can hold it up right away and i can use it to protect a gamewinning board of champion + other stuff getting pumped. It's also useful if you think your opponent just marketted for an azindels gift, as this is the only way primal can interact.
Feeding Time - Unconditional removal spells out of market are good. This is frequently the correct card to get in time midrange matchups, as pretty much all their threats can stymie you or snowball out of control.
Champion of Cunning - When you need a threat. It's really nice to get champ with both sides activated, and can really swing a topdeck war. Pay attention to how much influence you have, and be sure to play to activate it as best you can.
Black Sky Harbinger - I frequently market into black sky to close out aggro matchups. A black sky on 6 can be very difficult for red based aggro to beat, and it can be a second threat in your market against control if necessary, though obviously not at its best.
Cards I don't put in my market -
Maul - way too conditional and too downtempo. Not to mention temporal is dead (I hope).
Rain of Frogs - Like most effects of its kind, this card looks a lot better than it is.
Rindra's Choice - This card is not good. Do not put it in your deck. Do not put it in your market. Backwards annihilate does not kill enough things to be good.
So far as gameplay goes - try to figure out what role you play in the matchup, be aware of when you can go on the beatdown, and be willing to play for tempo. I think I merchant for permafrost to freeze up a blocker more than any other card in my market, and gaining tempo and putting pressure on the lifetotal is the key to winning midrange matchups with this deck. On the flipside- in aggro matchups the key is to trade almost as often as possible, and try to resolve an unanswered lifegain threat. The games aggro wins against me are almost universally games in which the aggro player manages to find a removal spell for a big lifestealing threat to tempo me out.
Ultimately I don't think crow is the correct direction for this deck, especially in a post temporal metagame, and i haven't seen temporal since the patch. I replaced them with rindra, and i've had some success, although i'm not sure how much of that is due to rindra and how much is just because I've drawn well over the course of my last 15-20 games. I'm currently sitting just inside top 20 on leaderboard, and I honestly don't feel like I have any particularly polarizing bad matchups.