Real short answer: It depends entirely on the state’s laws.
Slightly longer answer: In general, unless there are public health issues involved, the larger units of government (township? county? city looking to annex?) are not interested in paying for improvements unless the people actually want them, and in general, the residents are given an opportunity to approve or disapprove the proposed improvements.
But all that differs based on exactly what the state’s rules and regulations governing such improvements might be.
Better answer: Wait for plnnr to show up and address the question; he’s a pro. at this, while I simply worked for a planning agency, in adm. asst./paralegal capacities, for a bunch of years. I think there are a couple of other professional planners on the board too.
Suggestion: Form a residents’ association to be in position to jointly take the common-consent stance when somebody does actually try to do this stuff.
Final Comment: It’s highly unlikely that a developer would try to “improve” an existing subdivision – he doesn’t make money off infrastructure; rather, it’s a necessary expense for him. Where he makes money is taking 50 acres of unused pasture and woodlot and cutting it up into 1/4 acre lots, plunking down 200 new houses on them, building the bare minimum of roads, water and sewer lines, and amenities that the state regulations and the sales climate will bear, throwing up some impressive Ye Olde Phayque Englishe signage at the entrance that cost him $250 at most but which gives an aura of respectability to his former pasture, and selling the resulting properties for several times what it cost him to buy the land and build it. So the actual scenario you envision is unlikely – there’s no money in it for the developer.
What could happen, on the other hand, is that local wells and septic will be adversely influenced by development around the existing rural subdivision, necessitating public water and sewer for health reasons.
Final point: Unless somebody in the subdivision or somebody in accord with the desires of the subdivision residents owns those fields and woods surrounding it, you have for the most part no say in what’s done with them. (State open space or greenway laws, where in place, may help a bit.) Contemplate jointly buying up the land you want to keep as “buffer” to assure that it doesn’t get developed and impact you adversely.