I have not been impressed by D20 Modern, so I didn't look at D20 Past.
For Victorian games, the Deadlands D20 sourcebook is very good. I plan to just ignore the alternate history and run with a timeline closer to the real historical past. I even came up with a name for the campaign: The League of Somewhat Extraordinary Gentlemen.
A lot of D&D material has been published that is suitable for the Renaissance/Reformation era. The Iron Kingdoms covers the Colonial era. So it should be pretty easy to run D20 games in any era up to the 20th Century.
I've had very little trouble adapting the D&D classes to World War II characters:
* The British Officer was a Rogue with skills that included Knowledge (Shakespearean Literature)
* The Australian Soldier was a Fighter with proficiency in firearms instead of archaic martial weapons.
* The British Radio Expert was a member of the Expert class.
* The Hungarian Count was a Maverick (from the Deadlands D20 book).
* The Gypsy Burglar was a Thief-Acrobat.
* Because the story involved World War II characters going to Ravenloft, I determined that the Serbian Priest was granted by God a selection of Cleric spells and the other abilities of an 11th level Cleric, specially for the trip. Otherwise, a priest would simply be an expert.
(After the Serbian priest was killed, I was going to introduce a German minister who had the powers of the Paladin class.)
On my To Do list is to create a tech-level system for Runequest. I've already come up with "System D" with a description of each of 21 levels, from the paleolithic era to the era where technology provides godlike capabilities to everyone.
