I like the campfire idea. There aren't any other factions that have a "sight only" building, so that's an interesting concept.
Ahem, Dwarves with the observation tower. But the idea is great, and yes it should be an upgrade building because if it has no attack if can still use its abilities while upgrading. Great idea.
Edit: I will return to finishing my list:
Units:
Hand Axeman: Uses a hand axe. Promotes to Axeman or Spearman with upgrade 'Attach Blade to Pole'.
Axeman: Swings a heavy axe.'
Spearman: Attacks with spear. Promotes to spear thrower.
Spear Thrower: Fast moving, throws a spear a short range, with a slow attack speed. Promotes to Spear Launcher with upgrade 'Atlatl'.
Spear Launcher/Atlatl-man: Uses an atlatl to launch spears a long distance with a fast attack speed and projectile speed, and a powerful attack.
Sabre Tooth Tiger/Smilodon: Heavy and fast, quick attack with either paws or jaws.
Mammoth: Heavy unit that causes immense impact damage.
Rhinoceros: Heavier horseman-like unit. Attacks by ramming with head.
Torchman: Glows because of his torch, large sight range, attacks with torch. Requires upgrade 'Torch'
Buildings:
Pen: Produces Tiger, Mammoth and Rhino.
Mammoth Trap: Traps Mammoth for huge food boost.
Campfire: Huge sight, does upgrades 'Friction Fire', 'Fire Hardened Spears', 'Smoked Meat', 'Meat Preservation' and 'Torch'. (Maybe others) Promotes to 'Mountain Fire'.
Mountain Fire: A tall mound of earth and rock with a bonfire at the top, providing excellent sight.
Craftsman's Hut: Does upgrades 'Stone Knapping', 'Obsidian Blades', 'Copper Extraction', 'Copper Smelting'.
Upgrades:
Friction Fire: Decreases time required to make Campfire and Mountain Fire, increases sight of those buildings. Upgraded by Campfire.
Fire Hardened Spears: Increases ATK damage for spearman. Upgraded by Campfire.
Meat Preservation: Increases food bonus from trapping mammoths. Upgraded by Campfire, requires 'Friction Fire'.
Smoked Meat: Further Increases food bonus from trapping Mammoths. Upgraded by Campfire, requires 'Meat Preservation'.
Torch: Allows Torchman to be built. Requires 'Friction Fire'.
BTW I really like the idea of lots of researches being about fire. Fire was important to Ice Age peoples for many reasons.
Edit 2nd time: I am continuing my past work.