Siege Rhino isn’t going anywhere, at least not while he’s legal in Standard. One of the most pushed creatures we’ve seen, he’s been crashing in since the first day Khans of Tarkir was legal. It looks like he may have even picked up some additional tools to be even tougher to beat.
Sylvan Advocate may eventually take over as the 2-drop of choice for Abzan decks. The Advocate is a very reasonable option on turn 2 as a 2/3 vigilance creature since it can attack and still be able to block many small creatures in the format. As an offensive option, Sylvan Advocate is a little bit on the small side, at least before you have 6 lands in play. Snapping Gnarlid and Heir of the Wilds both threaten to attack for more damage on turns 3-5, but late in the game, the Advocate is still great while the others are more lackluster. The combination with Shambling Vents and Hissing Quagmire is what really puts Sylvan Advocate over the top.
Reflector Mage is such a great tempo card that it may be the perfect fit for Abzan strategies. Assuming the mana works out, a much improved Man-o’-War option is incredible for clearing the way to make sure undercosted Anafenza, the Foremosts and Siege Rhinos can continue to get in for tons of damage. Not being able to get their blocker back on the table next turn can certainly be the difference between winning and losing. It may feel bad to bounce an opposing Siege Rhino, allowing them to reset the enters-the-battlefield trigger, but if they can’t cast it next turn and can’t live until the following turn, things work out quite well.
Oath of Nissa is a tough card to evaluate. There is a small benefit to making it a little bit easier to cast Gideon, Ally of Zendikar, but the card is being played for its likeness to Anticipate. For reference, if you cast an Oath of Nissa off of a Forest turn 1 without knowledge of any other card in any zone, and with 21 creatures, 25 lands, and 3 planeswalkers, you are essentially a lock to not miss entirely (although that remains a tiny possibility). Being able to smooth out your draw and mitigate the chances of finding either the lands or the right creature you need is nice, but there is a very real cost in needing to pay that mana. Your mana base gets a little bit worse, although the deck already stretches to include Warden of the First Tree.
The jury is definitely still out on Oath of Nissa, but Sylvan Advocate and Reflector Mage are strong cards that are sure to see lots of play in Standard in either Abzan or a multitude of other potential decks!