Constantinople is way out, indeed. Greece could (and almost did even ITTL) get most of the Dodecanese islands with Samos, which were actually Greek-administered during most of the war. Thessaly might be a possibility only if the revolt goes much better, but there the Greeks would have to face Turkish cavalry, lacking cavalry of their own. Historically, the Greek regular troops, drilled in the European style, proved themselves able to withstand charges by the Turkish cavalry, but they were always few and not well-regarded by the ''kapetans'' of the rather unsteady irregular forces that made up the majority of the rebel army.
On Crete, the local revolt could probably succeed in surviving, but almost certainly not in winning, if given more support from the mainland. The main reason it failed completely was that in 1824-25, the Greeks lacked funds to actually launch most of their fleet at sea, and were riven by civil war. However, even in a best-case scenario, the rebels would dominate the countryside but fail to take the major coastal fortress towns (per every subsequent Cretan revolt). Given Crete's importance, not to mention its substantial (at the time a bit less than half of the total) Muslim population, it is most unlikely that it would become part of Greece. An earlier autonomous status however might be possible.
The main obstacle to any "more successful Greek War of Independence" is the Greeks' infighting, which led to disasters like the fall of Missolonghi, the civil wars of 1824-25, and the squandering of the British loans (which if concluded and exploited properly would have given Greece a most formidable navy instead of hobbling her with massive debts even before independence). If you manage to have a Kapodistrias-like figure (i.e. a virtual dictator) from earlier on to provide a coordinated effort, things would be a lot different, especially in how well the Greeks could stand up to the Egyptians. How long such a figure could survive before being assassinated like Kapodistrias, is another question.
Frankly, territorially I doubt there would be much difference from a more capable direction of their war. A clever diplomat like Kapodistrias might be able to get some more scraps in Epirus or Thessaly, but 1881 borders are probably a stretch. Where the Greeks would definitely benefit in comparison to OTL would be in bringing the war to conclusion earlier (1826/27 instead of 1828/29), without the massive destruction wrought by the Egyptians in 1826-28 and without the need for Navarino, which provided the pretext for Great Power intervention in Greek politics until the mid-20th century. Plus, a better-performing Greece would get OTL's Leopold I of Belgium as king instead of the imbecile Otto, and that's already a major improvement!