I'm wondering if a different wife means that Pyotr lives longer. That said, if the alliance with Prussia (and by extension Britain) holds, could he try (considering Louise of Britain might be too high profile for Elizabeth's tastes) for an English (one of George III's sisters) or a Prussian princess (OTL princess of Orange) for his heir?
As for bride possibilities, a princess of Hesse-Kassel (daughters of Maximilian) are relatively unimportant but well connected (sisters are a Prussian princess, a Holsteiner duchess and Peter's own sister-in-law, the princess of Anhalt-Zerbst, niece of the Swedish king and cousin of the prince of Orange and the future landgrave - both of whom are married to Hannoverian princesses).