Presumably he married her in the place of Henry of Champagne in 1192. Richard had at least one illegitimate child, so there's no barrier to fertility. So: if they have a son, he's going to be heir to the whole shebang of England and Jerusalem. Obviously, this would be completely ludicrous in the thirteenth century - even the Hohenstaufen's didn't manage it for long. I would expect 'King Fulk' (a proper Angevin name) to be imprisoned by some German Duke every time he travelled from one realm to the other, and all in all, this is a ripe situation for John to usurp the throne with popular support. Thus, England would be only slightly affected in the short term. Jerusalem, on the other hand, would get a decent King and the potential for a decent dynasty.
If Richard and Isabella only had daughters, they would succeed after the extinction of the line of Maria of Montferrat. In this case, the daughters would either return to England to try to regain their throne as they did OTL with Champagne, or John or Arthur would impose Salic Law with obvious consequences on the Hundred Years War, should it take place. Meanwhile, the daughters would marry the same sort of Levantine nobles as OTL, or maybe, if they're still claiming the English throne, one of the sons of Philippe Auguste. Then Louis VIII mounts an invasion of England when the Barons get sick of John (just like in 1214, but perhaps a little earlier - Alice of Champagne got hitched in 1210 OTL, Louis VIII in 1200, Philippe Hurepel in 1223) except this time, Louis has a decent claim to the throne and retains support even after John dies. He or his brother is crowned and hey presto, we have a new Capetian secundogeniture!