Pokémon will always have enthusiastic collectors (and these days in extension ‘enthausiastic’ investors/scalpers/etc.) for both ends of the spectrum, and the Pokémon TCG as a whole has the future. 
The fact that we are even having this pretty active thread and similar threads in the past, says enough imo.
As for me personally: I collect every era. I collect Pikachu and Seviper, the first spanning over every era and being released in all 16 TCG languages; I collect FA Supporter cards & other FA cards featuring people, which are definitely modern; I collect all raw, sealed, and graded variants of the WotC #21 Moltres artwork, which is definitely vintage; etc.
Lately I’ve also had a newfound passion for my Pikachu collection because of that. The very first Japanese TCG cards released on October 15th, 1996 were a Pikachu (and Jigglypuff); the very first English TCG product (Demo Game pack) of December 1998 included a Shadowless yellow-cheeks Pikachu; etc. Pikachu being the mascot of Pokémon caused it to have more than one TCG card in every of the 16 TCG languages; it has at least one card released in every year since the beginning of Pokémon (although not really evenly distributed, since there were just 10 across all languages in 2013, yet 233 in 2019 
); it’s cool to have a lot of the Pokémon history over the years seen in this single collection of mine. 
But getting back on-topic: it’s mostly certain individual cards that will have the highest values in the future, regardless of the era. Charizard/Blastoise/Venusaur/Mewtwo 1st edition Base Set holos are worth a lot more than Poliwrath or Hitmonchan from the same set; the Crystal Pokémon and Gold Star Pokémon are worth a lot; Moonbrean, the EBD trio, the Mario/Lugio Pikachu, etc. are worth a lot, to give some random modern examples. It’s mostly the eras in between (DP/HGSS/BW) that are underrated because of lack of heavy chase cards imo, except for maybe some high-end Japanese L-P promos. 
Greetz,
Quuador