Well, the example I gave above-- in the early Socket 754/939 days, ASRock sold a bunch of boards with an extra slot that would take a daughterboard that contained a Socket AM2 and DDR2 slots which would theoretically allow a significant upgrade on the "same" mainboard. Not sure anyone ever bought it, since it cost as much as a new mainboard.

The most famous example of this style of weirdness was the ECS PF88, which could be equipped with a Socket 939, LGA775, or a Pentium M depending on daughtercard choices.

But there was also some novel features-- motherboards with tube amplifiers on board (AOpen AX4B-533), a few generations of "instant boot mini-Linux environments", and some more sophisticated debug tools (I recall some firms trying small LCD displays and voice prompts to replace 7-segment POST code displays-- considering a 128x32 all-points-addressable OLED costs like $1 in quantity of 1, why are those not standard when the motherboard costs $300+?!)