Ignoring whistleblower protections, at least the whistleblower in some respects is personally involved, even if very indirectly.
Trump denies that Biden had anything to do with the hold up of Ukraine aid
Biden played no role in WH negotiations with Ukraine
If you want to investigate Hunter Biden separately, be my guest. But what does that have to do with what Trump did or did not do? If Trump denies that H Biden had anything to do with the Ukraine aid, how does putting him on the witness stand benefit Trump and the GOP at all in this impeachment trial?
It’s a whataboutism that Trump* uses all the time, and which his supporters poorly try to emulate, in order to muddy the waters. “But look at how bad they are” might work for people who already unconditionally support this administration, but the rest of us not in that bubble aren’t impressed.
Trump is being accused of trying to dig up dirt on Biden and wanting Burisma investigated for personal political gain.
If there is a legitimate reason to believe that there was corruption involved and that Hunter was involved, then that would help legitimize the investigation. The basis for investigating the Trump campaign by the FBI was reasonable accusations. If reasonable accusations were good enough there they might be good enough to legitimize an investigation involving Burisma.
Personally, I think that could backfire unless Trump’s attorneys know not only what they are going to ask but what the answers will be.