Future Community Day Potential
The starters aren't looking too good so far. Here's the outlook of all the starters past Gen1, and then some other potential picks.
Meganium: Still sucks with Frenzy Plant.
Typhlosion: With the same attack stat as Charizard, you might think it'll do well, but it falls behind even Heatran because of Ember.
Feraligatr: Will temporarily become the best non-legendary Water type, although Gyarados will still be competitive due to beneficial typing.
Sceptile: Will become the best Grass type attacker, but it still won't beat Raikou/Electivire or Kyogre in matchups unless if it's sunny out. Sucks to be a Grass type.
Blaziken: Blast Burn will make Blaziken the highest DPS Fire type, but it gets beaten out by Moltres with Sky Attack in the majority of relevant matchups for Fire types. (Grass, Bug)
Swampert: WG/HC will make Swampert a totally different type of Water attacker than Kyogre, offering more flexibility and a benefical typing to aid against fighting opponents with Rock, Electric, and Poison type moves. (Nidoking/Queen, Aggron with Thunder, Golem/Regirock/Rhydon/Rhyperior?) It also has a higher Stamina stat as of the rebalance, but I'm not sure how that effects its bulk.
Torterra: Will have a performance comparable to FP Venusaur, but by then it's already been outclassed by Sceptile.
Infernape: It may do better than BB Charizard and normal Flareon depending on matchup, but Blaziken is already better by this point. (And don't count Moltres out either)
Empoleon: Even with Hydro Cannon and great typing, it still under preforms, coming out worse than Swampert with upgraded moves.
Other Potential Days: Since the starters have already been covered here's some other candidates for a CD.
Salamence: Outrage would make Salamence powerful, maybe even topping Palkia and Dialga, but it's bulk would still be worse than Dragonite. Or it could get Twister, Dragon Pulse.... It would be a fine choice though.
Gardevoir: Is down but not out. This can still be the first opportunity to introduce a Fairy fast move.
Togekiss: Same thing, Niantic can use it to introduce the game's first Fairy fast move. It would also be really useful in a lot of relevant raids as it's even more tanky than Gardevoir. (Rayquaza, Palkia, Machamp, etc.)
Lucario: Perhaps Niantic could surprise us with Focus Punch, which could make Lucario a good glass cannon if it's better than the already existing best Fighting type move. (DP) After all, it is one of Pokemon's iconic squad.
Garchomp: Unless if Niantic releases a better charge move than Outrage, Garchomp will most likely be getting a worse move. (If it even gets a day at all)
Slaking: It could get Pound and redefine gym defense and become a great generalist. It could also be paired with Porygon-Z and make a "Normal type resurgence" day.
Flygon: Even with a somewhat broken charge move like Outrage, Flygon will always be a worse Garchomp.
Milotic: It already has the best Water type set in the game.
Rhyhorn: It would be a great pick, Rock Wrecker would obsolete Tyranitar completely! I'm not sure if that's a good thing, having been doing non-stop grinding for 3 hours last summer for Ttar...
To be honest I think they're running out of options. There's only so much they could do, and frankly, I don't think they'd do have the things on my list for whatever reason. And there's very few of these that are actually exciting for the commmunity, as they're outclassed and not worth grinding for.
Answers
Without abilities or status effects (and actual inmunnities that could make some typings suddenly usable in certain situations) the usefulness of a pokemon is reduced to its stats and moveset. And sadly since there's only one "best" moveset for each type you either get that or massive stats that can compensate worse movesets. That makes the optimal choices outside of legendaries really stale and as you said, besides hyping us up with new cool shinies, there are few pokemons that can turn the meta upside down at this point given new moves (or completely breaking it too)
Gardevoir/Togekiss: Fairy Wind will probably be the only Fairy fast move possible. While Togekiss can learn this move, Gardevoir cannot.
Lucario: I really hope for Aura Sphere which could make him competitive with Machamp. As Lucario's steel type cancels out fighting's weakness, he is going to be as bulky, if not bulkier, as Machamp.
Dragons: Dragon Rush could be the possible candidate for non-legendary dragons to close the gap between legendary dragons.
People keep saying this about Lucario but with bad defensive stats Machamp still pulls ahead as bulkier. For reference, it's similarly as bulky as Blaziken who itself doesn't fare better than Machamp despite taking less damage from fairy attacks. This was actually covered in GPs' Safest Bets article: https://pokemongo.gamepress.gg/safest-pokemon-bets-gen-4
Since it's brought up, the only damaging fairy move left to implement is Fairy Wind, a move Gardevoir cannot learn in the main series. To give it a fairy fast move would require adding a non-damaging move to the game or require converting an existing charge move to a fast move. Disarming Voice has been a popular pick for this. Togekiss could still totally get Fairy Wind though.
Potentially unpopular opinion here, I'm getting burnt out by CD moves being hyper meta focused. I liked Pikachu and Dratini days because their moves were novelty. As much fun as Beldum day was and despite getting treated so right by evolving an army of 7 MM Metagross with average IV's being in the 90's (minus the obligatory shiny) I thoroughly enjoyed the relaxing break that was Chikorita day. It not being hyper relevant meant I didn't need to pull my hair out over saving the absolute best IVs and if I missed one then who cares? I'm actually looking forward to Cyndaquil day being a sort of lull in the CD hype.
Like all glassy pokemon, Lucario benefits significantly from the rebalance and its beneficial typing already let it be almost as tanky as Machamp against fairy and psychic with the old stats. I haven't checked, but it could very well end up slightly tankier than Machamp with the new stats, so really all that's missing is the moveset with a good DPS.
I hope RNGesus sees my appreciation of Lucario here and gifts me a beautiful Riolu from an egg. Please ;__;
They are, but the old calculations I was referring to were done with Lucario's pre-rebalance stats. I checked, and with both at their rebalanced stats, Lucario is 5% tankier than Machamp if steel gives it an additional layer of resistance. That's all moves from the big 3 except EQ and most of Dialga's moves as well.
