I hope it isn't a Mythical or Legendary as I find those to be so freaking boring and useless. Who really gets excited for a Mythical/Legendary? Nothing more than box fodder.
I really hope it'll be a Pokémon from gen 8 like a new Eeveelution. I think the Pikachu and Eevee are going to be available in both versions.
Best case you would be able to catch Wild Pikachu and Eevee in Viridian Forest or get one from the building in Celadon like you got Eevee in Pokémon Yellow.
Sorta late to replying this (been playing a ton of Pokemon Go, walking about 20 KM will make you sleep early lol), but honestly I'd be very cool with a new legendary Pokemon now a days. Lately, I feel like gamefreak has finally started to consistently nail them competitively!
In the past, discounting the boxart legendaries (more on that in a bit), I feel like you're right: most legendaries aren't very good Pokemon. In gen 1, you got Zapdos who was good, but Moltres and Articuno had pretty awful typings. I'd say the 3 GSC beasts are sorta good now a days, but Entei and Raikou are pretty niche picks, and Suicune is basically an alternative bulky water. I should note though that Raikou has been pretty good in the past.
Then you got the Regis, aka the most screwed up trio ever. So they're supposed to be bulky right? OK lets give two of them among the WORST defensive typings in the game then! (Regirock and Regice). Registeel had potential, hell I actually used it in gen 4 Ubers for a bit, but its movepool sucks so much that I realized there were better Steel-types to be using. Also yes, I know Regice was OU in ADV.
Gen 4 is where I feel gamefreak started to actually try to make legendaries good at specfic things. Now, the pixie trio isn't really that special (Azelf is basically just a suicide lead, Mespirit is lame, and Uxie is a Memento + Dual Screens and Yawn spammer), and of course Regigigas is notoriously bad (I DID get top in the US with it in Festive Feud back in gen 6, but that's another stramge story rofl), but Heatran and Cresselia are excellent and unique Pokemon. Heatran has that godly typing with a good ability to compliment it, that also gives it reasonable weaknesses. Cresselia has Arceus level bulk and some damn good support moves for doubles, and it can be reasonable in battle spot singles (it was huge last gen, but Porygon2 has eaten a lot of its lunch this gen there), at the cost of meager offenses.
Gen 5 was pretty good as well about it. Like gen 4 it had a sorta meh trio (only Terrakion stands out here, and time hasn't been too kind to this thing with Fairy-types smashing it), but Thundurus and Landorus are pretty great. Thundurus actually got nerfed this gen with Tapu Lele blocking its prankster shenigans alongside Dark-types, but its still pretty usable since it outspeeds some important stuff like Garchomp, and Thundurus-T is excellent in Battle Spot Singles as an Electric-type check! Landorus-T is obiously huge everywhere it goes, its pretty much among the best users of Intimidate! Some people feel it's too common, but I feel that its just really splashable, not broken or anything.
Also, funnily enough, gen 5 probably has the worst boxart legendaries. Zekrom is sorta ok, but Reshiram is notrious for being the worst boxart legendary by a fair margin, Kyurem-White is also just ok, and Kyurem-Black... holy shit they mangled this thing's movepool rofl
Finally, gen 7 nailed it harder than any gen imo. Not only are all the Tapus good, but I like how their terrains can create different playstyles, such as using seeds in VGC. The Ultra Beasts are awesome too, with them basically being excuses for Gamefreak to give us wild ass stats and movepools you don't usually see on Pokemon. Honestly, the only Ultra Beast I think gamefreak flubbed was Guzzlord, whose poor defensive typing and lame stat distribution sorta mucks it up. Ok, I'll admit Blacephalon isn't very impressive either due to its poor movepool. Still though, they managed to make a workable Rock / Steel Pokemon! I can't belive that happened!
So TL;DR: I feel like legendaries have been bringing interesting things to the table lately, so I don't mind if Gamefreak keeps pushing them unlike the past!