The main thing that was possible was the ability to create and make new STC's, develop new tech and ideas and investigate the limits of technological advancements. Humanity could develop and grow and the technology could alongside them, innovation was encouraged and so new designs where being created all the time. IN addition Warp travel was discovered and travel to other planets in distant systems was achieved.
In the "Modern era" there are no newly created STC's there are no new technologies or developments, all there is are the rediscovery of technologies that have been lost, and many of these are misunderstood, the tech priests know they work, but not the how, or the why. The Adeptus mechanicus is prohibited from carrying out true development and innovation. Humanity has stagnated, or it could be argued is sinking backwards as technologies are slowly being lost.
If you take the example of the Imperial Knight. In the Dark age of technology the components that made it up would be intricately known and the way they worked would be fully understood. Therefore if one broke, needed maintenance, or was going to be upgraded or improved, it could be made as good as new on any forgeworld, and if it couldn't immediately factories could be built to allow it to be.
In the "modern era" alot of the technologies has become myth, the tech priests understand that you need to to certain things to activate and use it, but they don't understand the how or the why. They know that if this part breaks it can be replaced, but being able to build a new knight from scratch, or conceive of how the process would begin, without the correct STC template would be beyond anyone. The Modern knight vs the DAoT knight may not be technologically different, but, it would be better maintained, better understood and may, have better weapons who's STC has been lost to the ravages of time.
However in answer to your question, what was possible, "anything that an engineer could think of". Institutions wouldn't stop an idea from being created (hence Abominable Intelligence etc), whole worlds lived in Virtual Reality, Planets where terraformed and made livable in a way the imperium cant achieve now. But the main thing is that technology advanced and developed.