I was just thinking about this.
Say Garner squeak out a win, maybe Roosevelt has a fall that shakes people's confidence. Garner/Roosevelt win a modest landslide in 1932, but likely less that IOTL. Garner, crafty insider that he is, may give Roosevelt a more prominent role than the "buck of piss" he got - Roosevelt could be a very public figure for the Administration - still doing fireside chats and photo ops, which Garner plays the Presidential role.
Garner's election has been used as a way to make the Great Depression worse - if FDR is assassinated for example. But I do not think Garner would do nothing. The liberal and New Deal minded Joseph T. Robinson is running the Senate with an iron fist, so there will be action taken there. Additionally, liberal Cabinet members and other administration officials will take unsanctioned actions, reformers who want to get ahead even without total approval. Garner cannot veto everything his party passes.
What we may see is the delay of the Imperial presidency - with Garner taking a step back from the hand's on Roosevelt of OTL, letting Congress (which he has great backdoor influences on) have a longer leash, as they did IOTL up to this time.
So Garner has modest reforms to stop the Depression, which ironically may end up with the same final result that we saw with Roosevelt, given his recession in '37. So Garner could likely get re-elected with a modest win, Roosevelt still by his side, and have the Democrats well poised come 1940 - with perhaps the first time a Vice President running and winning - or maybe not...?
I doubt Garner would break Washington's rule of two-terms max.