Will tower unit have any cross over with valve
No. Valve’s property is their own and there wont be any copyright infringements on Tower Unite’s behalf. The workshop is another story.
It could be but like Drachen said it’s Valve’s property and they must allow it.
They did on others indie game like Super Meat Boy, Surgeon Simulator 2013, Poker Night 2 and I am Bread you have some stuff like the Headcrab from Half-Life series, Heavy from TF2, GlaDoS from Portal.
Would be interesting to see Valve work with them to make in-game items for both Tower Unite and maybe TF2, but PixelTail have only just recently became an actual company and the game is far from done.
Maybe sometime in the future one of the two companies will reach out to the other.
Valve tend to be quite open when it comes to their IP and I am pretty sure if Pixeltail were to ask them to put a reference, they would be quite open about it. Now the question is, what hat from TF2 would you like to see in Tower? I vote for the Ghastly Gibus for the irony of it.
Having the TF2 Mercs appear as buyable player models in TU would be fun.