Having read Beevors book about Crete i see that the allied forces could have defeated the German invasion with ease if they had gotten properly prepared.
WI Crete had held?
The immediate effect would be to strengthen the RN in the Eastern Med, since the losses taken to evacuate crete wouldnt happen.
The addition of Crete also allows the RAF to dominate the eastern med.
The stronger hold might allow the British to increase the forces they sent to the far east - although it might not make a difference.
In the Med, since Crete is part of Greece and IIRC there were greek troops evacuated there, as well as the local population, we could see some greek divisions in the Med. Which might lead to much more pressure to invade Greece (either before or after Italy), and the campaign going in via the Balkaqns rather than France.