I am thinking of a game where you start off in the caveman era, ( maybe add a few traits to the character making him harder to control and act more ape like or something ) and have to mine resources and build machines and structures that help you research and advance through eras/ages. The ages and rough dates will probably be like this:
Prehistoric 1000BC
Stone age 500BC
Bronze age 0BC
Dark ages 200AD
Medieval 700AD
Renaissance 1200AD
Industrial 1500AD
Electrical 1900AD
Nuclear 2000AD
Digital 2100AD
Nano 2200AD
I know they kind of bunch up into the same date at the end, this is mainly because of how much technology has advanced recently.
Oh and you won't go through ages as a day by day thing, you will have to research technology and when you get to a certain point ( or maybe number of technologies researched ) you will advance to the next era, unlocking new resources and getting rid of old, useless ones.
The landscape will be open world, and have randomly generated dynamic terrain ( meaning you will be able to destroy/edit it and hopefully build anywhere on it, although it would be much easier to have a tile based building area at the spawn as this would also stop the player going to far from the start and prevent the world from getting too big )
I am thinking it will have to be a side scroller ( even though aerial view would be easier ) because you don't find resources like uranium sitting around on the surface, and it wouldn't really be possible to do any mining if it was aerial view. On the topic of resources...
Wood ( prehistoric - industrial ) - building
Stone ( stone - medieval ) - building
Bronze ( bronze - electrical ) - crafting
Iron ( bronze - nuclear ) - crafting
Coal ( dark ages - electrical ) - upgrading/power gen
Gunpowder ( medieval - nuclear ) ( unless until industrial ) - weaponry
Gold ( renaissance - nano ) - crafting
Aluminium ( industrial - digital ) - crafting/building
Oil ( industrial - nuclear ) - power gen
Uranium ( electrical - nano ) - power gen/weaponry
Dark matter ( nuclear - nano ) - power gen/weaponry
Neptunium ( nuclear - nano ) power gen
From here on I will need some creative ideas of elements/materials that will be found in the future

I will need some more building/crafting materials
So why would you need to build? Firstly: to craft. Just like in Minecraft where you build a crafting table to craft, you will make far more complex crafting structures like ore extractors, robotic arms and more. Secondly: mobs - the main cause of lag and main part of what make a game fun....
I am not entirely sure whether I will include mobs, at least not in the first release. I will certainly try to make the game be able to support them so if I change my mind it isn't too hard to add them in, but the collision will be expensive and pathfinding is always an issue for me ( not sure why, I just find it really hard to get it right and not lag the ... out of the game )