Well, there are two main challenges when designing a shield for the trinket: the lack of real estate on the board, and the limited number of available pins. Those are amplified with the limitations of manual soldering, the minimal specs of common PCB manufacturers, the available packages of the chips to be used, and the budget limits.

For instance, if I wanted to make a shield with a LED matrix on it, my first choice would be the HT16K33 chip that Adafruit uses in their backpacks. However, that chip is easily larger than the Trinket itself! So instead I would need to look for something smaller, possibly in a QFN package, like the IS31FL3728. I will also need to choose chips that use the I²C or SPI bus, because there can be multiple devices on those, or otherwise choose solutions that minimize the number of pins used, like using the ADC to read the states of buttons connected to different resistors.

Connectors are going to be a problem, since most of them have huge 2.54mm pitch through-hole pins. The solution for this is to use SMD versions of those connectors whenever possible, and use of smaller connectors. For instance, for the PWM shield, I'm going to use 1.27mm pitch connectors, compatible with the sub-micro servos.

Finally, if nothing else works, I can grow sideways. making the shield more square, with room for the connectors on the sides. But that is the last resort.