So you can buy a new, nice looking shell, what is the big deal? A lot, under the hood.
At first, I was very disappointed that the Game Boy Advance does not have a backlit. I literally missed collectibles because I could not see those without a backlit.

So I bought an IPS panel. And wow, it is a totally different experience.
I also had to buy a new shell to fit the panel, but the old one was ugly anyway.

And next, I started to play on a headphone and boy o boy, the noise is bad. I am no audiophile but even my low expectations were not met. Turns out, the console board generates the noise. Long story short, dehum dehiss kit ftw!

https://www.retrosix.wiki/dehum-dehiss-removing-noise-from-gba-audio

The black thingy in the middle is this kit. You can find the connecting points by searching for crappy soldering 😇 and it really does wonders.

Now, with all that extra stuff put inside, the battery life is degraded. But as you might guessed, there is a solution to everything. More power (1500 MAH), less hassle with the USB-C charger.

Not all but most parts are from @retrosixuk , and I can highly recommend them if you are looking forward to pimp your retro console!