I don’t collect anything from WOTC so I’m intentionally leaving anything from them out of my list. Not to say I don’t believe they won’t rise in value, but I just don’t have enough experience with those cards to give a good opinion.
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Rare Promos (Japanese especially): Stuff like the Rayquaza/Giratina Lottery promos, PLAY Gold Stars, L-P lottery promos, and others are already insanely expensive and were released relatively not too long ago. Even common stuff like the Pokemon Center Shiny M Gengar EX has seen a decent price increase lately. I purchased a few of the Gengars at around $9 each when they came out, and recently sold a couple unsealed mint copies for $20. I think that as time goes on and it gets harder to find mint copies of these promos, the prices will skyrocket, especially for sealed ones.
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(Don’t hate @garyis2000) Gold Stars: I have seen a sizable price increase in Gold Stars lately, and it looks like the trend will continue. Certain ones may have secret hoards, etc., etc., but gold stars are 1) Extremely recognizable, 2) Very tough to pull (1/2.5 boxes, WAY harder than WOTC shinings), and 3) Unique. I think they will continue to rise in price, with PSA graded ones rising even faster.
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PSA 10 English Full Arts: The boom for Flashfire seems to be decreasing as more people are realizing how insane the payoff is to get a PSA 10 M Charizard EX Secret Rare, and thus grading more, getting more 10s, and driving down the price. I forecast a big drop in prices for PSA 10 MegaZards from Flashfire, before rising again, but never back to the $1000/card mark. However, I think PSA 10 Full Arts will hold their value well. It’s very hard to get a PSA 10 English FA, and recognizable ones with low populations (Charizard, Rayquaza, etc.) should maintain their high value and rise over the years.