I think once the game has taken off and gone live that there should be a graphical addon, ruins... Actual buildings or houses, stop signs askew, things that you would find after a natural disaster. Some of these buildings you can reuse the materials out of. A huge example of this is Battlefield Earth the Movie. Yes it was horrible but it still awe struck me, seeing the ruins of the city after man had been wiped out and survived extinction but was quite literally thrust back into the stone age. Overtime some....some basics should be added like electricity.
Electricity, the only true magical power known to mankind. It can be created but never controlled, nothing can stop it. The only other force of nature with similar power like this is steam. The ancient Mayan ruins even had signs depicting an acid battery which worked when scientists replicated in accordance to deciphered glyphs. Natural occuring metal elements when added to water would emit a glow or even produce electricty.
Steam: Steam should be introduced later in the game and used for driving gears or used in mass hauling, like trains for a big example. Though the tracks must be layed and can be damaged over time that lead beyond the tribes territories. This will allow for huge economical trade but the tracks must follow local waterways, locomotives need steam to run, steam as we all know, comes from water.
Sacred/ Forbidden Zones: Zones that are protected by a toxic mist/ or mutant beasts. These areas have rare items that will be necessary to technological advancements and feats. Granted one could bravely chart these lands and bring a few relics back but hard resources can only be packed out by means of mass hauling like trains. When the train is enroute to/from its destination any tribe can raid it/destroy it or take it over and kill everyone off and take the goods back to their tribal lands.
Flight.....hmm flight I think if its introduced into the game should take the longest to reach as far as advancements. Gliders are possible since they are a simple design. Helicopters and other things I think should be left out, unless the game transitions into a Divinci like age. Examples of games that used this idea: Rise of Legends, im sure there are others. The Vinci nations used steam to make machines and then were able to make tanks/helicopters/airplanes that used steam to drive them in the game mentioned above.
Ships are a no brainer, Ugg Ugg the caveman could make a ship. (more of a raft but you get the idea)
I have no doubt that someone else has listed these ideas already but I am bored and waiting for server up.