well, I'll go in order. There's always the option of having shadow Lugia, like a counterpart twin sister or something. A counterpart who's a futa to go at regular Lugia.
The lake trio could absolutely be a triplet scenario, maybe feeling what the others feel.
With Giratina, Palkia and Dialga, there's definitely potential when using their origin forms. Perhaps futa forms. Also, lots of fun space bending can be done with Palkia.
Cressilia definitely comes across as a lovey dovey type, but I could also see her to be a closeted sex freak. Plus, there's a lot one can do in dreams, cause screw biology.
With Heatran, despite being a legendary, it has a 50/50 gender ratio, sounds like a futa to me.
Regigigas seems to be far more advanced than the other Regis, so I can see her having more freedom with what she can say, information wise.
Reshiram and Zekrom are both dragon types, weak to fairy. Makes me think they're secretly weak to lovey dovey romantic stuff. Do what you will with Kyurem.
Swords of justice: just a bunch of dick jokes. "Give me your sword!"
The forces of nature are definitely muscular, but like the creation trio, they also have alternate forms. Plus, there's a secret fourth one, being Enamorous.
All put the other legendary gens at a later date. I'll list the mythicals next:
Manaphy is definitely into breeding, considering she already has daughters, being phione.
You already have an image of darkrai, so I'm sure you already got stuff planned for her.
Shaymin is a bottom heavy shortstack, also form shift.
Arceus is absolutely the kinkiest mofo of them all, being able to change types and all. She also definitely could somewhat change her body, being literally god and all that. Also, definitely joke potential: "Holy shit" "No. Holy fuck"
Victini is possibly very hyperactive, having infinite energy and all that. Rarely sleeps and is never exhausted, a real handful.
Keldeo is really the same with the Swords of justice, just inexperienced.
Meloetta, singing ballerina, as you stated, lots of interesting positions. Plus, beautiful moans and little concerts on the ship. She can change to her other form, which she dances in, do what you will with that information.
Genesetc is literally a cyborg, likes of fun stuff you could do with interchangeable parts. Also lots of personality potential. Having self existential issues like mewtwo did, so potential bonding for them.
Diancie is a very pretty pokemon, a mutant from Carbink. It's mega evolution looks like a wedding dress, wife material. She's probably very onto the idea of getting married.
Hoopa, as I said before, is a portal gloryhole queen. Unbound form also has potential.
Not sure about Volcanion, maybe something to show Groudon and Kyogre can get along.
Magearena is a robotic maid made for a king's daughter. Her type weakness is ground and fire, so probably really into being dommed by Groudon. She also has a original color form.
Marshadow is a copycat, loving to fight. So she would get along with Zapdos and Entei. She's also said to be associated with ho-oh. Also has her Zenith form.
There's more, list more another date.
For side pokemon, there's the horny pokemon of the Fandom, gardevoir, Lucario, lopunny, vaporeon, salazzle, Zoroark, Absol, etc. There's definitely other pokemon to work with, like Audino or Pacharisu, the latter referencing a vtuber named Snuffy when she was doing pokemon smash or pass with Ironmouse. Also, twin action with pulsle and minun.
Some humans: Nurse Joy, Officer Jenny, Hex maniac and more femboys. Goth and femboy make brain go brrr.
I can list more once you do the poll and know what you're thinking about.
Potential stuff: Nipple fuck + pregnancy, dick fuck + ball pregnancy, ear fuck, titjobs, assjobs, anal pregnancy, plus tons more.
Please do marriage stuff like you mentioned with that one human, emotional intimacy is amazing.
If you don't have one, make a discord server for ease of communication with fans, easier to get opinions and ideas as well.
Can't wait to see what you do!