I have a theory.
The reason 8472's dna is so dense is not because of their hostile fluidic environment, but because like most any known universe there did used to be billions of different sentient lifeforms, but they all got along really well and started interbreeding, and they kept interbreeding for millions of years until every one in that universe was exactly the same... They assimilate sexually. I mean seriously, your kids turns out to be 9/10s weirdly alien, what are you going to do? Maroon them on a jungle moon and never talk about them again?
The next logical two parter, would be a mixer, where Voyagers crew and a host of 8472 get drunk at a dance and pair off into dark corners of the ship.