Well, the problem with wondering about what the Republicans want to hear and who they want to bring, is that they have couched their entire defense off of:
--a thoroughly fake conspiracy theory about Ukraine and some server that was used to help Hilary--factually, this never existed. It can't even be investigated further at this point, because it is less tangible than El Dorado. Likewise, there is no human of any kind of merit that can speak to it in an actual Congressional hearing, where one is sworn an oath to testify about it
--attacking every single witness on invented character flaws, because their testimony is inherently damning, and they are unimpeachable witnesses to the crimes that Trump has committed.
...so there is yet to be any substance to what Republicans have presented to defend their president...so there really is no one they can bring forward to establish a defense, that we know of. The only important people that we haven't heard from yet are the ones that Trump is attempting to prevent from talking, refusing Congressional subpoenas. (all obstruction charges)...and yes: there's a reason he doesn't want them talking.
...of course, Trump could always talk if he wants to. It's a completely open door for him to show up and defend himself at this point. Even Bill did that.