Assuming that we make a house rule that contradicts Greysage’s answer for Is an attack of opportunity an immediate action?, would combat reflexes be able to grant additional immediate actions rather than additional attacks of opportunity?
Even if you ruled that an attack of opportunity requires an immediate action, the feat specifically says it grants additional attacks of opportunity:
You may make a number of additional attacks of opportunity equal to your Dexterity bonus.
With this feat, you may also make attacks of opportunity while flat-footed.
So under that interpretation it would effectively grant immediate actions that can only be used for that purpose.
If you did rule that it granted general-purpose immediate actions instead, that would make it incredibly powerful on casters that can make use of said actions (the same reason that the Ruby Knight Vindicator prestige class is so good, but even more so), such as through celerity.