What is a single word for the capability of a theory, for example, to generate further thought and theorizing?

Solution 1:

I would such a stimulating quotation is evocative (it evokes other, latent, concepts), suggestive (it suggests other ideas to you), or inspiring (it inspires you).

If you're trying to emphasize the productivity of the word, the sheer amount of other ideas it generates, you might use an agricultural metaphor, such as fertile, fruitful, or fecund (or, even more indirectly, bounteous or plenteous, or as a noun, a real cornucopia).

If you want to underscore that a particular work has already had a significant impact and influence on later works, you could say it is seminal, or, rarer and less strong, germinal.

And finally, scraping the bottom of the synonymic barrel, we have both batful (yes, seriously) and the archaic term uberous:

uberous: fruitful; copious; abundant; plentiful

Which, I guarantee you, is poised for a comeback.

Solution 2:

I would suggest thought-provoking.

interesting in a way that makes you think of new ideas or that changes your attitude to something [macmillandictionary]

stimulating interest or thought
[thefreedictionary]

What is most thought-provoking in these thought-provoking times, is that we are still not thinking.
– Martin Heidegger

Solution 3:

I think the term OP is looking for is...

reach noun 3 b. - range or scope of influence or effect. (thefreedictionary.com)

I can't find an actual instance offhand in the book I'm currently reading, but I'm quite certain more than once in David Deutsch's The Beginning of Infinity he makes the point that a "good" theory is one which has extensive reach. Consider this from a review of another of his books...

One feature he notes in explanations is that some are universal: they have unbounded "reach", much greater than the domain they initially described.