There was actually a TL, a short one, about this some time in the past few years. It was minimal, basically the event ("I have a terrific headache") where those were his last words OTL is mitigated just a bit and he hangs on in dire condition for six months or a year or so.
In my comments I figured the main opportunity for significant deviation from OTL is easing Harry Truman into the Presidency instead of pushing him into the deep end suddenly with no orientation. I think he did well enough OTL, but the benefit might be not so much to him (other than saving him some nasty painful stress, but Truman was strong and lived at least to 1968, I forget just when he passed) but rather mainly in other Democrats having more confidence in him. In particular I had the notion that his relationship with Eleanor Roosevelt and thus the left wing of the New Deal mafia would have been stronger. Also possibly FDR would fill him in on various unwritten deals, such as the promise to Britain that they'd share in the benefits of the Manhattan Project.
History is full of irony of course. Conceivably the Democratic establishment having more confidence in Truman might somehow cause him to lose in 1948, but aside from the fun of being perverse I honestly don't think so. It could rob Truman of his iconic David vs Goliath heroic image of OTL, but the likeliest outcome I think of mended fences with the Democratic left is a stronger united front for New Deal values that enables them to do more, maybe get national health, statehood for Puerto Rico, maybe other New Deal stuff Truman believed in by flipping one or more postwar Congress to the Dems (that is, preventing the Republicans from taking over, or flipping it back sooner and having that last longer).
Cold War is conventionally seen as what wrecked Truman. Indeed his preference was to stand down the military, not that he can be solely blamed, Republicans slashed military budgets as fast as Democrats did. He wanted more money for domestic New Deal stuff, and Republicans wanted to downsize the whole government. I'm suggesting maybe the Dems get to keep more ND agenda, and might come around on containment of the Soviets. Instead of Republican "I told you so!" if they open with serious desire to work closely with Stalin but Stalin keeps slapping them in the face the Dems might adopt a moderate containment policy, as Truman in fact did, but maybe earlier, with earlier anticipation of a need for somewhat higher force levels preventing the deflation of postwar demobilization falling down to the same nadir it did OTL, and with plans in place for force restoration should it seem necessary.
China is still going to go Maoist of course, I don't think anything the USA could do would stop that, maybe slow it or secure some corner of mainland China, presumably around Guangdong, as a Kuomintang bastion, but that would require massive American intervention in several forms--lots of money, lots of arms, and some commitment of American ground as well as air forces to actually fight on the front lines and assuming a line is held, stay on in allied de facto occupation forever.
This OP is more open ended, but I think it is egregious to mess with a second POD, such as throwing in a second Wallace VP term. Of course nothing will enable FDR to live on in health much longer than OTL but much earlier lifestyle changes which would certainly allow the 1944 VP slot to be open to almost anyone, but as noted also change FDR so much that his OTL standing would be in great doubt. I think it is implicit we are talking about Roosevelt much as we knew him, and that means his potential for living longer is very limited, and he should be making the same choices for the same reasons. So I think the old TL, titled something along the lines of "Franklin's No Good, Very Bad Day" is pretty much up to the reasonable specs of this OP--I think FDR's death was delayed into '46 in that, and if not a little tweaking to make his condition a bit less dire could accomplish it. Instead of that TL, we could instead ask what if FDR does not live out his fourth term but does serve in it as apparently fully capable and dies just as suddenly and unexpectedly as OTL, but later, after 1946 has started? Then poor Truman is once again pushed into the water suddenly, for I doubt FDR would change his ways and prepare the man. Unless it was as in the other TL, where he is stricken and he knows it and his wife and other insiders are conspiring to minimize the political damage, much as Woodrow Wilson's wife did.
As noted postulating a hale hearty FDR living on into 1949 and beyond is pretty out of touch with history, and postulating making it plausible by his living healthier when younger gives us a very different FDR in 1932, and makes the ATL very far removed from OTL even if we can get away with claiming he still gets elected 4 times in a row.