I would be opposed to requiring all techs in an era to be researched before a new era was opened. That said, you could require more prereq interaction among the techs to enforce general teching a bit more, instead of the beeline. Also, one thing I think Civ IV did better than Civ V was military prereqs were such that powerful new units required a lot of techs. Infantry for example required rifling, as did calvary. Thal fixed the infantry problem but there are other areas where you can get access to military units....even though their real life required technologies don't have to be researched.