A buddy and I went through and it seems very feasible that all of the new D/P Pokes plus their underevolved forms (Magnemite, Magneton, etc) are added to the Sinnoh dex. Altogether this makes 49 more entries (including Rotom) which leaves 10 left. This 49 includes Eevee plus every form from previous gens.
We then went through and came up with feasible roster changes to the gym leaders and E4, both for their first round and final rounds (after the National Dex stirs things up.) We tried to incorporate the new guys while keeping it logical. Pretty interesting, though truth be told likely way the hell off knowing GameFreak.
For the lulz I'll list them here, though I won't explain anything unless asked. Also keep the "WTF is wrong with you"s to yourself as this was just done, again, for lulz. I don't expect it to have much merit. Also these were generally founded on opinions we already had of the mons. What I will explain though, is the Eevees: My friend thought it'd be cool to have all of the Eeveelutions used the first time around, and the types kind of help accomodate this. Also, because we have no idea which gym leader is getting cut and how things are changed up, we just went with what was already there. Who knows if Volkner will be champ or whatever. All we know is that someone needs to have Garchomp & Co. for dex purposes, and thought it was feasible if Cynthia could be fought outside of the Elite 4 after facing them, which would then polish the dex off. Who knows. Lastly, Gallade in the 3rd gym is because using the same levels and move consideration, a lv27 Gallade is not all too threatening. Same for a lv22 Leafeon. Anything major that wasn't covered by a gym leader or E4 would be relegated to some ace trainer on victory road. Heh.
Rock - 1st: Geodude, Nosepass, Cranidos; Final: Aerodactyl, Golem, Rampardos, Probopass, Omastar, Tyranitar
Grass - 1st: Cherubi, Turtwig, Roselia, Leafeon; Final: Roserade, Torterra, Exeggutor, Leafeon, Cacturne, Cradily
Fighting - 1st: Machoke, Gallade, Lucario; Final: Machamp, Breloom, Blaziken, Gallade, Lucario, Toxicroak
Water - 1st: Gyarados, Quagsire, Vaporeon, Floatzel; Final: Gyarados, Vaporeon, Floatzel, Quagsire, [Water/Ice], [Water/Psychic]
Ghost - 1st: Dusclops, Gengar, Drifblim, Mismagius; Final: Shedinja, Dusknoir, Spiritomb, Gengar, Driflblim, Mismagius
Steel - 1st: Bronzor, Steelix, Empoleon, Bastiodon; Final: Skarmory, Steelix, Empoleon, Bastiodon, Metagross, Forretress
Ice - 1st: Sneasel, Piloswine, Glaceon, Froslass, Abomasnow; Final: Lapras, Jynx, Weavile, Glaceon, Froslass, Abomasnow
Electric - 1st: Raichu, Luxray, Electabuzz, Jolteon, Magnezone; Final: Raichu, Ampharos, Lanturn, Jolteon, Magnezone, Electivire
Bug - 1st: Heracross, Yanmega, Gliscor, Vespiquen, Drapion; Final: Scizor, Armaldo, Heracross, Gliscor, Yanmega, Drapion
Ground - 1st: Rhydon, Whiscash, Mamoswine, Hippowdon, Rhyperior; Final: Swampert, Flygon, Claydol, Hippowdon, Mamoswine, Rhyperior
Fire - 1st: Magmar, Flareon, Rapidash, Infernape, Magmortar; Final: Houndoom, Arcanine, Camerupt, Charizard, Infernape, Magmortar
Psychic - 1st: Espeon, Medicham, Gardevoir, Porygon-Z, Bronzong; Final: Starmie, Espeon, Medicham, Gardevoir, Porygon-Z, Bronzong
Champ - 1st: Umbreon, Togekiss, Lucario, Milotic, Tangrowth, Garchomp; Final: Couldn't come up with anything
Sooo yeah. Went back to the whole mono-type thing for the most part because we had the room to work with. Friend really liked the idea of Lucian having a P-Z. As I said, stuff like Lickilicky was "lol Ace Trainer." Lucian was actually hard to come up with but in the end decided those were better than what he had. And I'm going to love seeing just how wrong we were.