Actually the main form of "development" of material in many 20th century works is permutation -- an example can be found in Bach's sinfonia in F minor for keyboard. In regard to the Bach, the medium range planning is to use a ground figure against a very chromatic motive which undergoes some permutations. Of course, the main driver of the whole thing is canonic procedures. So, canon - ground - permutation at respectively the global-medium- and small levels of organizations.
Of course today there are algorithmic procedures being explored with the computation power of computers and application of late 19th and 20th century advances in mathematics which create interesting new forms which could be traced in some respects to very old ones. For a more intutive example, Phrygian Gates of John Adams and a good deal of early minimalism could be said to be based on the medium scale of organization variation form using on the small scale older methods of working out the elemental material (for example, the additive process found in a piece such as Reich's Octet has an incipient precedent in Sibelius' works). The radical thing about some minimalism is the work does not end in a traditional sense - it stops seemingly on no cadential point - or the whole journey to a cadential point or the actual time for the development processes to occur over a large span of time. Sometimes, it arrives exactly back to its start but eschews traditional means to establish a sense of ending (Reich's Clapping Music).
As an aside, what is fascinating is a few of Chopin's Mazurkas which just fall on a dominant or tonic but have no strong sense of ending. Partly it comes from the folk material he uses (certain types of mazurkas would have this harmonically static feel from circling two tonal points .. one examples -
) but it is novel he brings this into the concert hall and salons. In sum. to me form is what arises from simply planning a piece. For me, it can be just finding a few sounds you want to arrive at and explore. The actually journey, well, it doesn't seem too interesting if you stick with textbook examples. It is like boarding the safe and secure tour bus of the most popular sites of a destination and then return to you hotel outfitted just like your home. (And anyway, the term sonata in the 1st Venniesse school had never established a very specific form - it was quite flexible.. When the term sonata first arose, itsimply meant a piece where several instruments will sound together. Here is an example from CPE Bach -
.)