The real world Greek agreement had the Crown going to Ottos younger (but not his older) siblings and their lines in the event Otto failed to produce issue.
Leopold is the youngest of his family - would the line devolve to his elder brothers in the event of his lack of issue? This would mean Albert and Ernest as being third and fourth in the line of Greek succession at the point of Leopolds Greek coronation.
Ultimately for the timeline, it's irrelevant as we know Leopold has issue and his line continues to the present. But the Convention of London in 1830 wouldn't have known that - so a designated contingency would be required.
If Otto isn't given a consolation prize of a Grand Duchy - would that leave the Russians a bit miffed and could this then be what triggers the Prince of Wales being promised to a Russian princess, or one being betrothed to George of Kent/Hanover?
Do we then shift the Grand Duchies to Leopolds elder brothers Ferdinand and Ernst?
If Otto is in Greece, what stops there being a civil war the moment Leopold does something that some people don't like?