I think a direct appearance of our god/patron is unlikely. There's already like 20 gods to choose from, and the whole idea of patrons is they are typically unique from warlock to warlock.
What I think they could do is have us receive some special class-only side quests for the custom PC where we experience some vague visions in our sleep that is based on cleric domain or warlock pact. We wake up having been contacted, and then we can explain it to one of our companions and get tags so we can be a little more specific in the case of gods.
Similarly clerics, warlocks, paladins, and maaaybe druids should all receive some sort of message from their benefactor on our first long rest in which said benefactor at least makes it known why they can't help and/or guides us along the path they want us to go to remove the tadpole. Druids and nature clerics get directed to the grove/Halsin depending on where they are. Warlocks could could be warned against making a pact with Raphael. Clerics and paladins could be encouraged to reach Baldur's Gate ASAP so they can seek out a temple of their god where said god can enact their divine will more precisely.
It has to be kept vague because otherwise it'd be a massive workload, but interacting with your god/patron/trees is such a huge part of these divine classes that it'd be a shame to never have moments like these. It is also fairly jarring in the case of clerics who's god apparently doesn't care that their divine tool in the world is being corrupted by something else.