I actually thought about designing some tracks on the existing maps (a quick search of the forums reveals that this has already been done once about three years ago) and making a thread, where you could post times for both full AI and human crew. Since Muse added the option to play every map in practice mode, this seems like the more viable option compared to a custom-lobby-event at a set time.
This means, of course, that there sadly wouldn't be any actual races; it would have more of a time attack character to it, something you could do by yourself or with some crew in the off-hours of the Lobbies-of-Icarus-metagame to show off your aviational prowess.
This would also probably be the easiest "racing" mode to implement, since all it requires are checkpoints, which already exist in the piloting tutorial, and a timer. It could even be a direct addition to the practice mode as a form of advanced tutorial teaching the layout of the map.
There are, however, some general problems that I see in a racing gamemode:
- Monotony
It may not be everybody's definition of fun to spanner away at a single engine for a prolonged period of time (however crazy that might sound to some people).
Since gunners are practically useless (side note: "gunner" referring to every kind of gun-operating personnel in this instance) on a ship that has no other ship to shoot at - either because there simply is no other ship, or because the gamemode wouldn't allow combat - and firefighting tools being pretty superfluous in such an instance as well, there would really be no reason to not go triple spanner/mallet/buff-engie, with close to none variation between the crewmembers' duties.
Furthermore, to force every crewmember in a public lobby to do so, even those who just want to relax by blowing up blimps with big bad booms, is bound to be a torture for everyone involved, so such a gamemode should be taken as far away from accidentally popping up three times in the vote for the next map as possible.
An interesting track design featuring quick ascends and descends as well as acute turns at the end of long straights with walls to turn and smash yourself to death against, as well as making use of obstacles and hazards on the map would surely help to get the engineers off the engines once in a while to fix the balloon or hull, but would probably still not make crewing in a race interesting enough for the broad audience.
- Teams
A possible fix might be allowing the use of guns, but in the current state of the game there are only gamemodes with two opposing parties. Holding bigger races than 1v1 with a broader gun variety than just mines would require a free-for-all-type gamemode (which in itself sounds intriguing).
Come to think of it, racing as a team is possible, too, although more competing parties than just two seems more appealing to me.
- Asymmetrical Ship Designs
Another problem when allowing weapons in a race is the asymmetrical configuration of some ships, including the Squid, which seems the most appropriate for a racing mode. Since the Squid has only a gun on one side of the ship, the leftmost ship might have an unfair advantage; especially so since the Big Turn of the Aft Gun.
I don't know if it would be that much of a deal, honestly, but running the tracks clockwise so that the ship in the outer lane has arcs on the ship on the inner lane might help to balance this out.
That, and you could always run your Squid backwards, i guess...
Last but not least, the idea of having little challenges on the course as mentioned in the original post sounds interesting as well...it gives the crew something to do. I imagine some sort of "blimp biathlon" where you would have to shoot target dummies at a certain range could actually make a gunner useful in a racing mode.
...aaand there goes my thursday afternoon. Sorry for the wall of text.