In FRR, I am currently Legendary hunting, like I was during the post game of Revelations Emerald. Like I said, I have 6 badges and am about to face Blaine, before I do I have an option to catch:
*Articuno
*Suicune(both are in the same cave)
*All three golems
*Zapdos(Raikout is probably here as well)
After Blaine, you take your first trip to Sevii Islands if you recall, and that's where Moltres(and Entei, probably) lives. As far as I understand it, Mewtwo is in the same cave as he always was and the rest are postgame material on the other half of Sevii Islands.
Honestly, I like this concept a lot. One of the big problems of Kanto based games is that for all the choices you had in the midgame to tackle objectives in whatever order you wanted, there weren't that much optional dungeons. Aside from the three birds and then Mewtwo, there weren't any Legendaries to speak of either, catching Mew via Missingno glitch doesn't count. This is fixed with a mod like FRR, and now I have endless team variety, even multiple Legendaries of various shapes and sizes if I want to catch them. The golems even have their own unique little areas, mimicking the chambers you find them in when you play RSE using FRLG assets. On top of that, you get all nine starters that third gen had by this point, with Cerulean City giving you gen 1 starters, Celadon letting you catch gen 2 starters once you unlock Strength TM(as well as Regirock), and once you can surf you not only unlock the gen 3 starters but also all the Legendary Pokemon I listed above. I like the large variety of teams I can make, I like being rewarded for going to high level dungeons and I like utilization of abilities the game gives you in a proper way, these are all good gameplay mechanics. I'm even starting to get used to the pre Physical/Special split mechanics, honestly the game is much better now that I know how it works and can switch gears with future movesets.
One thing I will say about Revelations is that one of the reasons it took me so long to finish that one was because the game just dumps a dozen different Legendaries at you post game and it kind of just loses the momentum at that point. You can't use most of them in Battle Frontier, what's the point? Dividing Legendaries so that you can use some of them during the main story as a reward for taming them earlier than you're supposed to is a nice touch, it makes capturing the uber Legendaries like Metwo and other box Legendaries feel more special later on, another good gameplay mechanic. Way too many Legendaries are saved for the post game, where they don't serve much of a purpose, RGB had it right when they made the Legendary Birds into optional dungeons and FRR only expands on that concept. Looking forward to finishing this Romhack so I can go proper Legendary hunting and capture as many of each Legendary as I can, so I can have proper competitive stats without being limited to one per game. Now that is a true endgame reward.