In fact, "future" can be countable, as the OALD shows.
(OALD) 2 [countable] what will happen to somebody/something at a later time
- The company faces a very uncertain future.
- We must seize the opportunity to shape our future.
- This deal could secure the futures of 2 000 employees.
to decide/determine the future of somebody/something
- Recent events throw doubt on the president’s political future.
The alternate definition can't be that which applies in this sentence. Here is this definition.
(OALD) 1 the future [singular] the time that will come after the present or the events that will happen then
- We need to plan for the future.
in the future
- The movie is set in the future.
- at some point/time in the future
- I don't expect any of these things to happen in the near future (= soon).
- Things will continue as they are for the foreseeable future.
- the immediate/not-too-distant/long-term/distant future
- Nobody can predict the future.
- What does the future hold?
of the future
- What will the cities of the future look like?
- Kevin talked about his hopes for the future.
We see that the article "the" is compulsory when using the word in this sense. When the word "future" occurs before a noun, the word is the adjective, which does, however, correspond to the meaning in "1". (at a future date, future generations, their future expansion,…)
Should we use "an" "because it's a non-defined/non-specific future i.e., one of 'any' futures, which is uncertain"?
I believe that to be the reason, except I'd formulate it slightly differently, since "future" can be used but only as either in sense "1" or "2". "Future" in sense "2" is more aptly defined in the SOED in the following way.
(SOED) 2 The prospective condition (spec. a succesful, prosperous one) of a person, country, etc.
Whereas "future" in sense "1" is not a prospective set of all the events bound to happen but the set of them that must result, "future" in sense "2" is conceived only as a possible set of future events, but restricted as concerning a small part of the world; this is done in the light that we know that in this particular case (persons, nations, organizations, etc.) the human factor plays a directing role which allows for a certain measure of choice in the shaping of these events. We can say that the future will comprise, among other things, all such prospective conditions imaginable that will be realized in it.
You can't do away with "an".