A few rambling thoughts:
- A runtime-reconfigurable CPLD. It starts up with the image last flashed via a dedicated programming connection (say, JTAG) to its internal flash. Groups of pins and cells can be locked into fixed configurations via security bits in the flash. It is through this initial image and a non-programming connection that you reconfigure the remaining cells (say, via SPI). The locking protects the runtime changes from being destructive, such as outputting on a pin that on your board goes to another output. Specification is open enough that you don't need a specialised tool to make these runtime changes.
- A conductor/insulator/conductor "sandwich" with a module coming out of the side that you could attach two jumper wires to. To use, you desolder an IC lead from its pad, insert the "sandwich", resolder, and now you can insert something in series between the IC lead and the pad (eg. to measure current). Bonus points if it comes with a tiny DIP switch that you can switch to keep it connected when not in use.
- Readily available extremely short 2.54mm female/female headers to adapt male headers/jumpers to male headers/jumpers, as well as allowing you to adapt a through-hole component to a male header pin where distance matters (eg. crystal, cap on a breakout board).
- A Surface mount 2.54mm male/female header that is insulated underneath, with a small side tab that you could solder to an existing populated SMT pad on a PCB, to add jumper-wire-friendly connection to a prototype PCB that you left out. I have no idea how you'd secure it for disconnections though.
- Minimal through-hole breakouts customised for each MCU produced with space to put caps and crystals optimally as standard. The design is released so anyone can make them. Purchasable assembled and unassembled.
- Some of the more interesting battery charger plus voltage regulator ICs with proper leads rather than being mostly leadless.
- Fully-assembled boost converter module as a through-hole component with no layout requirements. Five pins: In, out, ground, enable, and a resistor to set voltage.
- Small breadboard with no valley so that you can plug a two row 2.54mm connector directly into the middle via male headers and have connections heading outward from each individual pin.
- A breadboard that can grasp short pins.
- Jumper wire with small (eg. 22 ohm) series resistors located near either one or both ends.
- A small inexpensive reprogrammable eight-pin device that does nothing but apply a lookup table to a fixed number of input pins to produce output for a fixed number of output pins. Pins might be VCC, GND, three inputs, two outputs, programming pin. Like a small primitive CPLD.
- Power-pooling IC that accepts a few power sources and outputs uninterrupted power even as the sources are connected and disconnected. One source assumes a single-cell Li-Po and minimises the voltage drop. The battery source is isolated if the other sources are providing power. An output pin indicates if we are running from battery or not.
- An external power detect/LED IC for projects that use a battery. Overvoltage-protected input detects external power. Output (eg. to MCU) to indicate when external power is detected. Open drain output to connect to a LED. Input (eg. from MCU) that overrides the LED output, either on or off. IC is ultra-low-power when LED off and no external power.
- Shift register where every two bits controls what is connected to one pin: Either nothing, a weak pullup, or a weak pulldown.
- Inexpensive through-hole P-channel MOSFETS (if it's 6c for SOT-23, why am I paying $1.60 for something comparable through-hole?).
- A micro USB B connector with long leads for hand-soldering that is actually sold anywhere.
- Small SMT crystals with leads or at the very least hand-solderable pads on the side of decent size.
- A small surface-mount component (0603 or much smaller) that conducts as shipped but you can deliberately break the connection with a small tool, magnifier, and steady hand. Essentially a tiny dip switch with default on.
- A module that you can solder on to standard footprints that contains an upper module with the same footprint but with longer pads.
- An adapter that has 2.54mm male header pins on one side (say, 2x5) and individually spring-loaded pins on the other that will make contact with a wide range of PCB hole sizes. The header pins then go to a cable. For testing before soldering real headers on.
- Jumper wire with one end having some kind of connector that lets you make secure, but temporary connections with a plated through-hole of various sizes *vertically* at most points on a PCB.
That was actually quite fun to write up.
I'm sure quite a few of these exist already.