From Cambridge Online
We use suppose, supposing and what if + present verb form to make suggestions about what might happen:
What time shall we meet?
Suppose we meet downstairs at 4 o'clock?
When we are less certain, we use suppose, supposing and what if + past form to talk about future possibility:
Suppose we asked Mary to baby-sit? Do you think she’d do it? (not as certain as Suppose we ask Mary to baby-sit?)
Supposing someone else wrote the essay. How would we know? (not as certain as Supposing someone else writes the essay …)
When we refer to something that did not happen (something hypothetical), we use the past perfect:
Suppose we hadn’t brought our umbrellas. (We did bring our umbrellas.)
Supposing they had closed the road. Would that have been a good idea? (They didn’t close the road.)
What if I had accidentally told Maria about the party! That would have ruined the surprise. (I didn’t tell Maria about the party.)
Suppose we meet in the offices downstairs at four o’clock?