I don't think it's really open all that much quite yet. Depends on what else comes in Ixalan but Ramunap Red definitely stays alive after rotation
Very true; Ramunap Red is well positioned for rotation, as is Temur Energy. Both are fast enough to mean lifegain is an effective way to combat those decks, especially if you build a utility sideboard to match up against the most played decks. Tapping opponent's creatures with
Authority of the Consuls would be a good white sideboard card against either of those two. The main thing to defend against when competing with Ramunap Red is Hazoret, and
bomat courier can also be a weak point if they risk tapping out.
But there are also several other decks that will be viable after rotation besides Temur Energy and RR: UW Approach, UB/
Grixis Reanimator, UR Prowess, GR Ramp, there will be a planeswalker/vehicle deck I think, and all the new builds with new cards. Instead of 1 or 2 top decks there are 4 or 5, and no single deck is ultra-powerful in such a way as to prevent weird, spicy creations from coming out of nowhere (the renewed health of standard is shown in that mono red aggro can be competitive, as RR came out of nowhere recently). Good compared to recent standard iterations, but I'm sure it seems small coming from legacy and all its options.
Here's hoping there are several interesting cards yet to be revealed in Ixilan!