Things to watch for next:
Now that Trump has been hospitalized, the next big sign post is whether or not he get moved to the ICU. That would be significant for two reasons. First as a signal that the illness is progressing, and second it is at that stage that he would be expected to temporarily give up his duties as President and have Pence take over command responsibility.
It is worth examining how this process played out with Boris Johnson the PM of the UK. Johnson was admitted to the hospital after his illness was not responding to treatment. Within 24 hours of entering the hospital, he was transferred to the ICU. At the time doctors said this was a precautionary move. He was never put on a ventilator which does tend to corroborate the view that his move to the ICU was precautionary. Once he moved to the ICU however, Johnson deputized his Foreign Minister, Dominic Raab, to take over his functions. Ultimately after a week in hospital Johnson recovered enough to be discharged and resumed his PM duties.
In our case it appears that Trump's illness has not progressed as far as Johnson's at the time he was admitted to the hospital. Thus it would be surprising if Trump was moved to the ICU in the next day, or even two. Therefore if such a move actually happened, it would be a significant negative - certainly not priced into markets or political expectations. All the more reason it would be a huge marker.