1. Latios is useful for DB/DC to clear gyms if no legacy Dragonites. Otherwise no use for either of them, outclassed by other options.
2. Probably no. Regirock and Regice are decent TDO options for their types but DPS is more important.
3. Moveset won't affect Mew, it's pokedex filler even with a good STAB moveset. 210 base attack is way too low to be of any use.
4. Same stats as Mew ie. will be dex filler regardless of moveset.
5. Same stats as Mew and Celebi, same story.
6. Depends on how they implement the forms. Might see niche use as glass cannon in the attack form.
Looking to the future- unreleased legendaries and mythical pokemon use
Considering that with new quest system, we get chance to encounter and catch mew, and also encounter legendary pokemons, i think it is good idea to look at this who we don't get yet. My thoughts:
1. Latios and Latias- which moveset will be better; 2x psychic or 2x dragon? Also, if they have any use?
2. Regi trio- best movesets are obvious, but they will be useful?
3. Mew- i hope Niantic change him movepool and add at least one STAB fast move, if they don't do that; only pokedex filler.
4. Celebi- Have 2x psychic moveset, but... Confusion and Psychic... I think only Mewtwo is good with this moveset.
5 Jirachi- Confusion+ Psychic for psychic type specialist with the same problem as Celebi, or Confusion+ Doom Desire for neutral dmg. If he get steel type fast move...
6. Deoxys- don't know. He has 4 form and even in normal form has highter attack stat than mewtwo, but his def and hp are REALLY poor. Best glass cannon in game?
What are you think about unreleased legendaries and mythicals pokemons?
Answers
The only useful one will be Latios.
Latias is more on the tanky side, you're better off using Dragonite.
The Regis have low attack, but solid moves. They will fit well into last slots in teams, but who is going to spend a lot of rare candy on a last-slot Pkm?
Mew, Celebi and Jirachi all suffer from their balanced stats. Just look at Poliwrath for example.
Atk Deoxys is probably fun to mess around with, but its low survivability will make him unviable.
With current movesets neither is impressive.
Latios with dragon claw can be used for gyms, but not exactly cut for raids.
With weather boost and enough raiding regirock can supplement golem with its pure rock typing and massive defense
Mew has a wide variety of subpar moves. It’s amazing to see how none of them is good for anything
Celebi is decent at best
Jirachi has doom desire going on, coupled with eps from charge beam it can deal fast steel damage, or confusion for a slower experience
Deoxys is way too glass. And psycho boost may not carry it to top of dps charts despite high base attack. 70dmg for 4 second long attack isn’t exactly good