What we need is for testimony that Trump, himself, said Ukraine would not get aid until the investigations were done, and/or that the investigations would help his campaign or sink his political opponent. Those would be smoking guns.
We've pretty much gotten that from Holmes though based on his conversation with Sondland.
The GOP defense for why Trump held up aid was because he has sincere concerns about corruption and generally doesn't like giving out foreign aid. While the latter is true, the President doesn't have the right to nullify aid on a whim since the Congress has power of the purse strings, not him.
The former reasoning as was described in Holmes testimony also isn't true. If Sondland confirms Holmes' testimony or the other people who overheard this conversation corroborate Holmes' account it would be the clearest evidence yet that Trump's primary motivating factor in regards to Ukraine was dirt on Biden.
The only other persons of interest here would be Mulvaney, Perry, or Bolton in order to further hammer home the point. But we have direct knowledge of Trump's intent through the closed door testimony yesterday.