I have been trying to find evidence of a system of government where one of the principles it works on would be to hinder the creation of effective political families (thereby allowing people to advance based on merit and not familial relations -- though I accept this would not be a natural consequence of such a system).
The dogal election system in Venice was the first possible option, and that seemed to be close although not satisfying the conditions of this question as people from the same family have inherited the title after each other: List of Doges of Venice. The election system, however, is still fairly complex and does seem to be successful in curbing the power of families. Would anyone be able to offer comment on whether the fact that people from the same families inherited was due to those people being 'ordained' by the family or actual abilities they had? Was being the doge something a Venetian even worked towards (or would they have preferred another office)? [What about the other mercantile republics of Genoa and Amalfi and Pisa?]
The Papacy is another (and, quite possibly, the strongest) candidate. However, both Roman noblemen (earlier periods) and the Italian families of the High Middle Ages seem to have exerted considerable power in choosing cardinals -- or, at least in making certain someone would not be chosen. How effectual was this way of curbing familial powers, and how easy was it for someone to make their candidate the Pope? This will have surely also depended on the time period, with more recent elections being subject to stricter rules so highlighting the change in this would also be helpful (and why a change in rules came about to begin with).
The Polish-Lithuanian(-Ruthenian) free election is another option, but that does not quite qualify as the intention of every magnate was to find a weak king. The family of the person was less relevant, although certain familial relations may have been helpful for some candidacies.
Can anyone suggest other states where the leader was chosen (either automatically, by the system in place, or 'manually') with the intention to prevent the formation of powerful dynastic blocks?
Perhaps I should add for clarity that I do not think it necessary for one family to be disqualified entirely after one 'term of office' but rather for there to be a considerable gap in it -- say that a grandson could rise to the office the grandfather held, but that the father would have been ineligible to serve in any such capacity.