Let's say Tony Stark could create a new element, or (re)discover it. In the movies, (Iron Man 2) he discovers (or rediscovers as he puts it) a new element on the upper end of the Periodic Table. This is, of course, in a quest to replace the Palladium in his Arc Reactor as a fuel source... one that isn't going to poison him.
What is the feasibility of this fictional occurrence?
Well, as Jarvis indicates, the element Tony Stark discovers is on the upper end of the Periodic Table. Seeing as it's a new discovery (at least keeping in canon for Iron Man), it's safe to say he discovered a super heavy element, which would make for an excellent fuel source (ultra dense) on the Arc Reactor.
However, on the upper end of the Periodic Table there is a stability issue with super heavy elements. What would be the candidate for that new element then?
Interestingly enough, if you search for possible answers online, you find many conjectures about fictional elements and such (like Vibranium). But there actually is an element which would fit the needs of Tony for that Arc Reactor to replace Palladium, and interestingly enough it has a weird sort of history.
If we are to assume that S.H.I.E.L.D. knew about Vibranium but kept it under wraps, and we take into account that Howard Stark (given the time frame he existed was the height of those top secret projects and "alien technologies") It could be a given that Howard Stark likely had some involvement around the period's version of S4 Facility or Area 51. If you're dealing with alien technologies, then that's where your playground is at...
If you follow conspiracy theories, then Bob Lazar is a name you probably know well. While I'm not going to state that his claims are true or false, he *did* make an interesting claim to the effect of the element used as the fuel source for extra terrestrial vehicles at the S4 facility.
That element is now known as 115 Ununpentium.
However, it doesn't stop there... after all, 115 on its own is unstable. It's not like you can use it as-is for a fuel source. In order for this claim to check out, Lazar had to have been indicating a specific
configuration of Uup which is 299Uup, with 184 neutrons. To date, this doesn't _officially_ exist and maybe 4 atoms of this stuff have officially existed in raw form before it decayed.
But _theoretically_ 115 would be very stable at 299Uup with 184 neutrons. Lazar did indicate in the 1990s (or maybe 1980s, I can't remember) that the stability of super heavy elements is expected around 113-116 range whereby a configuration would be available. Ergo, the discovery of Unumpentium in 2004 is interesting as element 115 and theoretically in a very stable configuration of 299Uup.
So we get a claim by Lazar about the S4 facility having about 500 pounds of Element 115, wherein it would have to be as 299Uup for that to hold up. The connection to the 299Uup is that it is claimed as the element fuel source (isotope) for what would be alien space craft is even more interesting since those craft are (again supposedly) quantum gravitational displacement engines... which actually _would_ require a ridiculous amount of power to run and possibly an ultra-dense material for fuel.
The correlation here is back to the fictional Tony Stark and discovering a super heavy element for use in the Arc Reactor as a fuel source. Whereby if we look at the properties of the fictional Iron Man suit we realize there isn't much difference between the arc reactor and the abilities it allows the Iron Man suit... Repulsors and Flight... and the use of 299Uup in theory for the fuel source of quantum gravitational displacement engines at the S4 Facility allegedly harboring alien spacecraft.
Those same quantum gravitational displacement engines would be something that Howard Stark had access to during his time, as we saw in the Iron Man movie for the Stark Expo (R&D for the flying car on stage). Which would be a great cover for having a half-working system that need more time... indicating Howard Stark would have been part of the team (obviously) reverse engineering the original Roswell Spacecraft and it's gravitational displacement engines.
Ergo, Howard Stark wouldn't be able to say anything about the discovery of 115 and 299Uup as the fuel source for those engines, but he would encode it into the design of the Stark Expo (as indicated in Iron Man 2).
In the nd, it seems the most plausible element for Tony Stark is therefore 115 Ununpentium, 299Uup as it is an element which would (at least in theory and conjecture) harbor all of the properties associated with gravitational displacement engines and ultra-dense fuel source which also exists at the far end of the Periodic Table (as Jarvis says in the movie).
It would also explain the apparent difficulty inferred by Jarvis about constructing that element (in a stable form). The next predicted element for stability like Ununpentium in 299Uup for is somewhere around the 243 range and would be exceedingly harder to stabilize than Ununpentium on the periodic table, so while I agree Tony Stark is a super genius, I think he's likely working with 115 and not 243 for the sake of stabilization.
Interestingly throughout all of this, I get the weird feeling that Lazar might not have been totally bluffing after all