So can hope powers "believe" things into existence ? we've seen Jake do this with brain dirk, along with creating that pumpkin thing. if so than can all hope players use hope to believe in something so much that it comes true ? can hope players believe in an object or idea to the point that they become tangible ?

Only certain Classes or under certain circumstances. Think about it, even with the immense amount of Hope Jake was producing, he was only able to make Dirk be real for a couple of minutes, and it also took him a long time through his power-up to actually make Dirk non-fake. Pumpkins are way easier to create, and he was being amplified by Trickster Mode. So the answer is ‘not all Hope Players’ and ‘It’s still a god damn hard thing to pull off’