This would probably be way more complicated than it seems, and very possibly not be viable, but...
It would be cool if:

DF was able to build light parties with 0-2 additional dps players and the mobs in the dungeon simply gained 30% extra max hp for 1 additional dps and 60% extra max hp for 2 additional dps.

So if there were 4 dps and 1 healer queued just waiting on a tank, instead of only 2 of those dps players getting in when a tank queued (and the other 2 having to wait for another healer+tank) it would just throw all 6 into the dungeon and all mobs would simply have 60% more hp than normal. If only 3 dps were queued up when a tank+healer became available, it would throw the 5 of them into the dungeon with a 30% boost to mob hp.

Allowing for variable Light Party size like this could really go a long way to fixing dps queue time. As I said at the start though, this might be a lot harder than it seems. And ONLY tweaking mob hp to compensate seems like the way to go but it might have unexpectedly bad results on certain scripts/mechanics.