is there any alternative OS to give more lifetime to an older iPad? I have an old iPad that was stranded in my possessions a while ago. It has iOS 9.xx installed and won't update any longer. Also more and more Apps are stopping to work because the newer version won't run on 9.x OS anymore.

Considering the state of the planet on the whole it would be nice to being able to install any sort of alternative OS to such a technological masterpiece in nearly mint condition and likewise give it a few more years of being useful.

Can anyone advise whether there is any project that would be worth looking at in this regards?
As far as I know: Not really. 
All you can do is to jailbreak it - as @Nimesh Neema suggested -, which is usually done to do some customization of iOS. It shouldn't allow you to install apps that require a newer iOS, yet you can install apps outside of the AppStore. I am pretty sure though, what you are looking for, does not exist.