Based on his initial witness list, it looks like Schiff might abandon the entire quid pro quo argument. It was never supported by the facts. Plus the names of the first witnesses released so far likewise do not support this contention. In addition to that, the rumor is that Schiff had the "whistleblower" in for a formal interview, and even Schiff thought he sounded over-the-top partisan. Can you imagine how bad that guy must be in person for Schiff to schiff-can him?
Trump: No quid pro quo.
Pence: No quid pro quo.
Pompeo: No quid pro quo.
Zelenky: No quid pro quo.
Volker: No quid pro quo.
Yanovitch: No quid pro quo.
If this turns out to be the case, then they will go for the more general abuse of power/obstruction of justice. If you go with the theory that he will put his strongest witnesses on first, then he thinks career diplomats will be able to sell their case that Trump should be impeached because they dont like his "America First" which they have spent decades fighting.
We will see how that goes.