How would you go about remaking Ruby/Sapphire/Emerald

Also, there is a big difference between being in OU and being something that every single team needs to have a solution for. Reuniclus was OU but did in no way have as much usage nor did Reuniclus define BW OU the way a weather inducer like politoed did.

If you don't have a solution for something, it's going to severly cripple or outright beat your entire team every time you see it. Every team didn't have a solution Reuniclus, but those same teams were losing to it at team preview. CM Reuniclus in particular was one of those pokemon that would beat your entire team if you didn't carry something to prevent it from setting up and/or something to 1-2HKO it. Lacking a check, counter, wall, or revenge killer for any pokemon used frequently enough to be OU is bad team building

Not to mention tyranitar is still one of the most used mons in XY, so is excadrill.

Excadrill is one of the best rapid spinners in the game, if not the best. Mold Breaker is its most common ability by far, likely because of Rotom-W and extremely short duration of sandstorm.
TTar has been OU since GSC and gets buffed in one way or another every generation, I doubt he'll ever fall from OU hehe. Among many other things, TTar is a Talonflame check, a SR setter, a powerful pursuit user, a ghost resist, and a special wall (especially with assault vest). Unlike last generation, though, his sandstream provides much less support for pokemon other than himself (SpDef boost). So much less that building a team around it is gimmicky, at best. The same can be said about Politoed, Ninetales, Abomasnow, and MegaCharizard Y and their respective weathers.