The first question becomes whether he'll exit as a free agent. His contract sets up a ping-pong match that starts with the Eagles exercising a $20 million option for 2019, Foles exercising a $2 million buyout, and the Eagles then deciding whether to apply the franchise tag, which would cost in the neighborhood of $25 million for one more year.
The Eagles, if they tag Foles, presumably would then try to trade Foles, who may not be thrilled about the team showing its appreciation for his contributions by trying to squeeze something more for his rights than the compensatory draft pick consideration they'd get. But they'll understandably hope to guide him to a team other than Washington or the Giants, a pair of division rivals who could become very interested in pursuing the Super Bowl LII MVP.
Only four other teams would seem to be obvious potential suitors: The Dolphins, Bengals (maybe), Jaguars, Raiders (maybe), and Broncos. Everyone else seems to be set at the position, and most of the teams with older quarterbacks (like the Patriots, Steelers, Chargers, and Saints) would presumably throw the incumbent overboard only for a much younger player.
The lack of a long list of no-brainer options for Foles could put the Eagles in a delicate spot, with the $25 million tag applied to Foles and then accepted by Foles, making his salary fully guaranteed and forcing the Eagles to keep him around for another year, with Wentz making peanuts in comparison under the fourth year of his rookie deal.