I'm not sure if this was asked before, I have checked here, and haven't found anything related, so apologies if it was previously mentioned.

Would it be possible to add an internal CD display for trinkets?
I'm totally clueless when it comes to programming of course, so I'm really not sure what it takes to make that work, but basically once a trinket like, lets say, [Heart of Rage] - http://www.wowhead.com/item=65072/heart-of-rage procs, it takes 90 seconds until it is available again. It would be nice to see that on the bar itself like the other OmniCC CDs.

Since (as far as I know) each trinket has a different ICD, it might require some work to maintain a database of trinkets and their ICD. In that case it might be a good idea to create this feature and let the user add his own CD.

You should try this addon:

https://github.com/Resike/InternalCooldowns

I don't think this is something OmniCC itself should do, since it requires knowledge about the source of the cooldown that OmniCC doesn't have. What should exist, however, is an addon that updates action button/item cooldowns based off of some database of that type of information. If that is written, then OmniCC will display the cooldown as text.