I like the idea of MB being a totaly end game content for few best players, but I realy think that requirements should be different for Diamon and Platinum. Don't get me wrong but double experience and +1 to catch multiplier gives huge advantage after you put it all together.
In my opinion experience requirement should be at least 50% higher for Platinum (with exp ticket Diamond has x2 rate and Platinum x3 so 50% more sounds fair to me). Not only you get experience faster but Platinum is also less populated what makes it much easier for people to find a decent place to hunt in peace.
I have no idea how to fix catch matter, it doesn't make much of a difference for very rare catches since both worlds most likely have to get to the bonus rate but starting from rare the easier catch it is the faster you will get it on Platinum what makes grinding to 650 a lot less time consuming. At this point I don't even feel motivated to try catching when I see people throwing 2-3-4k balls on some random "rare" pokemons that doesn't even have bonus rate like Skiplum, Flaffy or Nincada.
About limiting catch possibility, I don't realy thing that reward for such high effort achievement should have any limitation other than 1 use per character/account. Saying that "mythical" pokemons are going to be excluded is like telling me that Catch Charm > Master Ball. 0 effort to get it (just cash) and time to spend on the spawn. Sometimes a lot of time like in Billy or Khormes cases but surely less time than getting all requirements for MB.
Overal the whole idea sounds realy interesting and it is something that could make me start playing seriously again but not in current state.
I totaly agree with years of "playing" is not a valid requrement. Too bad noone thought about it when Mega Stone achievement was added so now people who have a friend account that was AFK for 9 years is considered more loyal to the game and can get nice rewards but people like me who got almost 5 years of constantly putting premium on accounts got middle finger and 4 more years of waiting. But nothing can be done about it so ignore this part, just a little "cry" here.
I agree with there being no restriction on masterball. I mean isn't the whole point of masterball is catching the Pokémon you always dreamed of?
I also agree with there being no year restriction, if someone is playing hard enough they deserve the masterball (and imo the older achis as well but that ship has sailed).
What I don't agree is your suggestion of increasing the requirements of Platinum for the achievement related to masterball. Yes we have it easy but that is the point of Platinum having higher rates, we left Diamond/Pearl becaus we wanted things to be a bit easier.
You are saying it is easier for us to exp because we have fewer players but keep in mind that our economy is very, and I mean very bad and because of that, we cannot find the TMs we want for our Pokémon most of the time. You can say that it is unrelated but for example I for one usually don't like using 130 lvl Pokémon to exp my character, and I know many people that are just like me.
Thank you for acknowledging the fact that we can't get the VR catches easy as well (I for one caught most of my VR catches after extra).
I've reached extra on many "rare" Pokémon, even ones like Octilerry and Piloswine, I also used over 1k ultraballs on Croconaw (sadly my old pc died so I cannot give you the correct number), I also used 1.4k gbs on skiploom when I was playing on Diamond/Pearl (can't remember which since I switched my world many times with my character Dnkboy), so I understand your frustration, I mean that's why I am playing at Platinum right now.
I hope noone tries to take my comments and try to turn this thread into a war between Diamond and Platinum. I just wanted to give my opinion.