This wont be everyones cup of tea due to the rough kitbash I did and my below par painting skills but might spark some ideas and shows that, with a bit of imagination, and some extra bits you can totally maximise your kit potential.
What you will need:
Tervi/trannofex kit
Some extras from either a Tyrant/Carni/Nid kit
Green stuff
So basically I began by building this guy - a full Tyrannofex kit as I saw him evolve
With the extra bits left over, specifically his brood nest I wondered could I build a Tervigon from the extra bits.
I began by glueing the brood nest together
I then added the second head from the kit to the front where the Termagant would usually spawn
With the extra crushing claws from the unused Tervi/Trex kit I stuck them to the front
I then kitbashed a tail from extra bits
I then added one of the Trexes defensive claws to his back and an extra crushing claw to his rear from a Carni kit
Finally I added two front claws from the Tervi kit to his sides and glued under him the spawned Termagants
Here he is fully painted (basic enough paint job as Im not a great painter)
And a Trex side by side comparison
I need a base for him though, any ideas on where I can get one,
GW dont seem to sell them individually?
Hope it sparked some ideas!