The return to Sinnoh in Brilliant Diamond hits the nostalgia spot while also feeling new and different. While everything is in the order I remember, things have changed with new moves, more customisation and slight changes to a few things.
Battling is largely the same with the turn based, element based moves forcing animals to beat each other into unconsciousness. A few generations of games have gone by so there are attack moves I didn’t recognise that Pokemon can learn including Fairy type.

At the start it’s still fairly pacey, leaving the house, going in grass and accidentally stealing Pokemon to battle with. Team Galactic are interesting foes that are trying to capture a god of time to harvest energy and that’s alongside the player having the goal of beating all the gyms to challenge the Elite Four and the champion.
Experience share and capture experience make this game a bit easier than the original. However I do now like the idea of capture experience because it helps with the goal of catch-‘em-all which previously went against gaining power. We also now get effects from Pokemon happiness and friendship values as they will survive attacks, resist status effects, dodge things and get more critical hits based on the values. I do think it’s a shame the happiness system wasn’t more fleshed out with mini games or something as you don’t have to do anything to make Pokemon close or happy.
There were also extra features in the old game. Poffin making, exploring the underground, Pokemon shows and a safari with non-traditional capturing still feature in the modern game but because the DS was different to the Switch these things have changed for the worst in my opinion. The old touch controls made them fun and entertaining, the modern controls are mostly stick and button based.
The player character can change its outfit in some ways and there’s a choice of appearance when starting a new game which is new.
HMs were removed since the original Diamond so the strategic decision of bringing a level 3 Bidoof along has been removed. The choices and slight puzzles that came with HMs are made pointless as you can use the effects no matter what if you have the correct gym badges. This is worse in my opinion.
Another thing that really irks me about this game is the amount of special evolution Pokemon. The ones that need trades, stones or happiness are all over the early game which is probably why players often end up running the same teams so often. Not being able to get the fully evolved versions of Pokemon is quite annoying.
I also find the art style odd with the big heads and tiny bodies on the people. Other than that it looks like a graphically updated version of the original Diamond. The appearance is a downgrade from Sword though.
I’ve never bothered with much past the champion so I can’t comment on post story content.
Sinnoh has my favourite starter Pokemon and is quite difficult compared to a few Pokemon regions I’ve played. The Elite Four are still challenging without lots of healing items and the game itself is one of the better ones.
Pokemon games in general have a problem with the two releases meaning the Pokédex is impossible to complete without copies of the other game.
As a Pokemon battle focused game it is quite brilliant.