How about take shape?
assume a distinct form; develop into something definite or
tangible. Example:‘the past few months have seen the state's health
insurance legislation begin to take shape’
Synonyms: become clear, become definite, become tangible, crystallize,
gel, come together, fall into place.
The Free Dictionary lists takes shape as an idiom, which is kind of like one word ... if you're willing to stretch it. If not, I'll go with gel, which hasn't been mentioned yet.
Addendum: More examples from OD in response to @Tonepoet's comment. These examples generally show that taking shape occurs over time, gradually, slowly, in the OP's sense. Taking shape does not occur immediately or abruptly; it implicitly requires time, as per emphasis added and prevalence of various forms of "begin".
‘The party, now in its 12th year, helped residents build a sense of
community back when the development was just taking shape.’
‘The question that remains, however, is what alternative narrative
takes shape where the developmental trajectory of the narrative of
self-making leaves off.’
‘As we closed in on the far shore, a small town began to take shape.’
‘Alongside the secular model of marriage, an ecclesiastical model is
beginning to take shape and definition.’
‘A reversal of that conventional wisdom began taking shape in the
early nineties, following the 1989 Exxon Valdez spill in Alaska's
Prince William Sound.’
‘Elation replaced doubt when the schemes began to take shape.’
‘While compositions on their previous effort had time to develop and
take shape, here they are set to impact at once.’
‘Here's a look at some of the other developments taking shape around
‘Globally, ambitious efforts to develop wind power are beginning to
‘The outlines of another strategy also began to take shape.’