xmarksthespot
CEO, BS Comics
The four confirmed Omega mutants are:
Quentin Quire
Elixir
Phoenix
Iceman
An important thing that Elixir shows is that the power does not necessarily have to be destructive/offensive in nature.
If you want to define what should be considered an Omega mutant you should instead consider what the common features are between these four mutants.
Imo defining Omega has two criteria - type of power and level/potential of power - and both must be fulfilled in order to qualify.
Type of power:
Psionic manipulation of one or more (forms of) components of Marvel reality - matter, energy and consciousness.
Phoenix - matter and energy in general, and consciousness.
Iceman - kinetic energy
Kid Omega - consciousness
Elixir - organic matter, biochemical energetics
Level of power:
Their powers have the potential to expand in such an exponential fashion that their limits - if they do indeed have limits at all - to what they can do with their powers and to what scale they can use them are so far above other mutants that they effectively have no limits when compared to other mutants. The four confirmed Omega's are at different stages in their powers.
Phoenix has achieved her full potential.
Iceman has achieved some of his potential.
Kid Omega has to an extent as well.
Elixir hasn't yet done anything that impressive.
With younger mutants there is more leeway for them to be later revealed as omega mutants. For the old guard I'd say it is unlikely e.g. Xavier is imo a very powerful alpha mutant but not an omega - he's lived about 60 years(?), his powers are pretty much honed to their attainable maximum.
Three of the four have also shown the ability to transcend a corporeal form and exist as a a self-sustaining consciousness that may inhabit inorganic matter and/or energy (with Elixir being the only exception). So the ability to do this as power develops may be a requirement as well. All of the mutants acquired their powers naturally so imo mutants that have been enhanced by technological means should not be considered.