hmm I'm actually a bit unsure what to do normally I avoid using a pokemon that I have used on the first playthrough in the same generation. However witht eh weak trainers and prevalence of easy experience my teams quickly became too strong so I got a new pokemon to train thus building several teams through the game in both X & Y
Unfortunately we are left with a limited selection of pokemon even worse quite a few good choices in hoenn are already taken
Hoenn Dex Pokemon Used in X&Y:
Flygon, Salamence, Crobat, Blaziken(the speed boost mega one), Vileplume, Sharpedo, Manectric, Alakazam, Aggron, Golem, Gyrados Azumaril, Starmie... That really limits my options in game ORAS...
As of now I do anticipate using both Latias and Latios on their respective versions. For starters I'm planing Mudkip and Treeko (as Torchic was used this gen) I also expect to use Breloom and Whismur and quite possibly Spheal or Absol (which were both members of my Emerald team) I may also send over an adaptability Corphish and Tropius may see a stint on my team though I doubt it will get much use outside of fly, sweet scent, cut and strength... Poor thing back in ruby it had at least one free move slot as sweet sent was useless outside the safari zone. I will never forget that it was Tropius with a combo of solar beam and fly who won the battle against Steven when Groudon and Blaziken were out for the count... (For the record steven was down to his last mon which wasn't even steel type... they should have at least added more steel types so Steven could actually be a steel trainer...)
An other choice would be to join the swarms of hoenn and raise a Tentacruel(just like every swimmer ever lol)
I may also bring stuff from X & Y over from outside hoenn including perhaps mega stones
If there's one thing I would say to the people who are going to transfer over pokemon from another game, it's that it might ruin the gameplay. In pokemon White 2, I transferred over a number of pokemon not found in the game and my final team ended up being Emboar, Torterra, Milotic, Staraptor, Haxorus, Lucario. This honestly destroyed the gameplay of the whole game for me because of how unnatural it felt with the game. That's why this time I'm only using pokemon that can be found in game in Alpha Sapphire so I get the "full" gameplay experience. Just a word of warning. It could be that no one else has felt this way and that its just me lol
I guess that may be a problem for some but generation 3 changed that for me with quite honestly one of the worst species distributions in the series.
HGSS also changed that as the playthrough were I sent over a few eggs to start was one of my favorite playthroughs and the Arceus event was a fun addition with a baby Giratina and some of those event pokewalker pokemon.
while I agree with you for new generations in remakes I don't like being restricted to old options only as I've already done that...
But its all opinion anyway ^_^