It'd be really interesting to have visceroids (and perhaps other tiberium life) available. Perhaps they could have a chance to spawn as a neutral (read: hostile to everything) whenever someone is killed with a chem sprayer. I'd say they should have a chance when someone dies to Tiberium toxemia, but that might lead to a really stupid tactic where people deliberately kill themselves in the opposing team's Tiberium field to cause a distraction for them.
I liked the idea of having a possibility to become one in a crate, but if you're an expensive class, that could be kind of a bad thing. (Crates kinda need to be reworked, anyway.)
As for their actual dynamics, they should have very high health, zero armor and heal in Tiberium. They could spawn more AI-controlled visceroids with a Tiberium spray attack, but that could obviously have balance issues. They'd be good against infantry and light vehicles, doing moderate damage to heavier ones. Possibly, they could have a melee attack as an alternate fire that would do more damage.
For a really offbeat idea, perhaps they could be available as a character class to buy at a terminal in the silo, only available to whoever currently controls it. Perhaps at some far point in the future, Forgotten mercenary hubs could become available to give the controlling player access to specific characters and ghetto (but useful!) vehicles.