class Alpha:
[default] Item(index):
get:
return DoSometing(index)

No special syntax and semantics needed.

C#'s way of doing this(index) is a very nice syntax.

It's clear and to the point and you'll know what to expect even before you know what an indexer does.

class Alpha:
self[index]:
return DoSomething(index)

And non-default indexed properties will switch over to square brackets instead of parentheses too:

class Person:
Child[index] as Person:
return cast(Person, _children[index])

And manual use of [DefaultMember] attribute will be deprecated.