I spent some time thinking about this and trying to figure out how to keep the spirit of drafting from different eras, while still letting the draft function as easily as possible. Things became complicated when we decided to still allow trades like we normally do.
I came up with, what I think is, a pretty fair solution that is easy to work with.
Essentially, you are free to draft and trade anyone you want with two stipulations. . .
#1. You never have more than two players from the same era on your team at one time.
#2. You MUST have one player from each of the 5 eras in your starting lineup and one from each of the 5 eras on your bench.
This makes it super easy to check to see if a trade is legal or not, AND it forces you to still be strategic in who you get - but it allows more freedom to pull off trades to fill out your roster.