will pokemon rare cards be worth more in the future or go down or stay about the same?

There is no way anyone can know for sure

if you keep the card good in plastic, it will be worth alot more, google it of the value now

I'll bet not worth a damn. My brother traded a **** load of his first gen cards for 3 cards he didn't have to finish his first gen set. 

The value of items (like Pokémon Cards) will vary greatly depending on the number in existence & the desirability of the item.  The general trend of items typically runs in a "U curve", where the value of an item will initially fall for some time as quantity is high, demand is low & desirability is being still being evaluated.

It is possible that some may artificially inflate prices by creating an false demand, sequestered part of the supply OR participating in a fad, which can cause a severe depreciation when the truth is discovered or the fad dies.

After some time (depending on the item), the value will eventually bottom out at some point BEFORE starting to rise.  As long as the Pokémon TCG remains relatively popular (as it has a 20+ year history at this point) & the cards in question are NOT reprinted, they will appreciate in value over time.

it depends on the relevancy of the item, both in the past and in the future. a superman comic from 1983 would be super rare, but it completely depends

There is no way anyone can know for sure

Like all collectibles, it depends.  Those that paid hundreds for beanie babies years ago have stuffed animals work a few dollars each.

You can't predict how long Pokemon interest will last and if years from now there will be interest in it.  The best market for collectibles are those old enough to have disposable income that want to reconnect with their childhood.  Or, a franchise that keeps the interest up, such as Star Wars.

at least here in japan, they stay about the same as new ones come into market, believe it or not, Nintendo Japan keeps working on the franchise. But nothing like the original Pokemon ones, I even purchase Pokemon themed masks for public transportation