Here's how I see develop:
Open with develop, and develop an expensive card into 2 cards that *end up on the top of your deck*. This would give you a very accelerated opening.
So with a 3/4 opening, you might end up with develop and your $4 card colliding. Develop the 4 card into a great 5 card like witch/mountebank/etc. (Scheme would make this much worse.) Hopefully the strength of playing the 5 early outweighs develop's weaknesses. If there is no collision, then at least you can hopefully do something good with estate-->early silver.
Late game: I think that develop is terrible in the late game unless there is some cute+powerful combination like King's Court + Mountebank/Torturer/etc. Its power comes from developing an expensive card... otherwise it is the terrible version of remake. Its benefit in the opening may make it good enough that it is ok as a dead card in the late game (e.g. like how chapel and remake are usually dead cards in the late game).
---
Develop/baron opening: This is not so bad since baron tends to be a dead card later on. Hopefully there are good $5 cards for the baron to turn into. But even market is not terrible as this (I am just guessing) will be slightly slower than smithy/big money. Treasury is obviously better than market.
---
Obviously develop depends on having suitable cards on the board.
$4 cards to open with: baron, nomad camp (NC may make more sense as 3rd buy since develop is likely coming if you didn't draw it; or buy it with both the 2nd and 3rd buy), mining village?
Probably doesn't work:
Potion: If there is a compelling potion card like Familiar, you might as well open silver/familiar. If it's something like university, then the develop/potion opening may not be that awful. If you get the collision, kill the potion for the $5 you wanted. Otherwise you are trashing the estate for a silver and follow the university strategy.
Sea hag: This only makes sense if there is a $5 curser and Scheme on the board. Otherwise just try to get 2 sea hags and 0 develops.
Trashing cards like bishop, remake, trader: On one hand, you can argue that having 2 trashing cards isn't that good a strategy. On the other hand, a develop can do exceptionally well in a small deck with many expensive cards.
$5 cards: Aside from the top-tier $5 cards (e.g. the attacks), mint and mandarin are good to see on the board since develop lets you turn them into something more useful.
Trading post is good to see on the board.
---
Cute combinations:
9/7:
with Peddler on the board, peddler --> platinum is always good. develop Silver --> hamlet + $4 village (or other card/action and +buy card) will help pickup peddlers.
7/5:
king's court + torturer/mountebank/witch/rabble/etc.
forge + duchy may be ok in late game when you are rushing for VPs
6/4:
border village + almost anything (especially if governor is on the board)
border village + remodel --> province
border village + remake??? (right away you could remake BV into forge/KC, though remake is not that useful later in the game)
goons + throne room (kind of)
grand market + throne room (kind of)
grand market + conspirator
hoard + bridge (+great hall; because otherwise you normally want to buy a single really good Victory card as opposed to two, unless you have 2 hoards)
hoard + mining village (might as well sacrifice the mining village for a province or duchy instead of estate)
nobles + any village
gold + potion --> hopefully buy possession
5/3:
scheme + powerful attack (in early game)
scheme + minion (as a general strategy)
village-style card + card-drawer (torturer, etc.)
minion + menagerie (as a general strategy)
minion + fishing village and other disappearing cards (e.g. oasis)
library + disappearing cards
tunnel + discarding-card like Inn
menagerie + Inn/etc.
cartographer + wishing well
develop + any $5 to turn into a $6/$4
4/2:
tournament / haven
treasure map / haven (open with 2 develops, turn the estates into silvers... maybe hit treasure maps once or twice)
conspirator / any +card/+action
native village / bridge (as a general strategy)
crossroads / $4 victory (silk road, gardens, island)
crossroads / scout
island / hamlet (kind of; hamlet lets you avoid terminal action collision)
---
The best boards to open with develop (as far as I can tell):
Strong $5 attack + King's Court
Strong $5 attack + Scheme
Border Village + preferably $4 village or other non-terminal (or island)
Peddler + peddler-enabling $2/$4 cards. (and maybe $5/$3 enablers like market)
Minion + $3 cards that help minion strategies
Trading post on a board with engine cards
Hoard/bridge/great hall
Marginal boards:
University
Treasure map / haven
Tournament / haven? (haven't tested this)
Big money $5 card (library, vault, embassy)
Tunnel + $5 discard (Embassy, Inn, etc.)
Chapel? (use develop to rebuild a chapel deck)