If I could invent a new element, it would be one that ‘fissions’ like Uranium so it can be used in a nuclear power station, but which has no side-effects like radiation or a long half life. That would give us loads of free energy without the stigma of nuclear power and that would change the world!
If I were to have discovered one in the past, that would definitely be Gallium. It’s one of very few elements that are liquid near room temperature, it’s VERY unusual in that it (like Ice) is less dense when frozen, so solid gallium floats on liquid gallium (most solids are denser so sink: ice and gallium are WEIRD). Gallium also paved the way for the invention of very important technology we use today such as light emitting diodes (LED lights) and solar panels. GO GALLIUM!