This project actually started last fall, so I wanted to start with some entries to bring any interested readers up to speed.
The reason they had asked me is because I created some fireball launchers for an "interactive game"(some might compare aspects to a haunted house. I can share more about that if people are interested) several years ago. At the time, I created air cannon based creamer system. So they knew I was the person to ask, and they wanted something "cool" at the wedding.
First steps were of course research. I was quite familiar with the aircannon creamer systems I did before(still have them), and though they do create a very ideal display, they are too temperamental. The pilot light sometimes goes out, wind will prevent ignition, and sometimes just random luck will cause it to fail. The solution there was to have 3 launchers, and if some didn't go off, it wasn't an issue as long as one did. That system was good because it was quite safe, and inexpensive. Here, I cannot really afford failure, given that it is part of their ceremony. This site is much more wind prone than the site we used the creamer system at.
Web research was done, I talked with a number of members of a pyro community for ideas. Other things that were considered were a "balloon" with flammable(but oxygen deprived) gas, fireworks type systems, classic cremora's, among others. Many of these had reliability or other safety issues.
In the end, the route I decided to go was basing off the relatively simple propane poofer. Research suggested there were a number of things that could be tweaked to cause the display to be more like what was desired than the loud almost flame thrower effects commonly seen at burningman and many fire arts expos.
Next up, design and safety research.