Within the development of the Tyranid race in the 40K Universe, has come multiple Hive Fleets.
Within the mods of DOW, I believe this could be better represented in the modding of games, by having the capacity to change hive fleets, and not just stick with one fleet, Negating true evolution within the Race.
It wouldn't take much to implement a Hive fleets call button to represent the evolution of the specia within the game play.
With two generations of Tyranids available already, it would seem easy enough to do for those two Hive fleets.
The converted DOW 2 is from a different Hive fleet compared to the ones developed from the Tyranid mod.
Then it would just be a matter of the earlier two Hive fleets. Behemoth and Kraken.
To understand I think you would have had to have been there for Behemoth and Kraken, Also around during Leviathan.
I have also seen many smaller or splinter fleets.