1. I think the new game should limit itself to the GC. Nobody plays anything else anyway, now does anybody? Later starts can be offered as DLC.
2. It should start 1500, and go to 1740. That's the time-frame the game can represent best, anyway. Potential for add-ons:
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Frederic the Great's ambition: gameplay extended to 1781.
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Age of Venice: 1453 game start.
From there, just do the EUIII drill.
3. Keep it simple in the beginning. Generic Soldiers. Potential for DLC:
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French Pikeman
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French Arquebusier
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French Ship
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French Louis XIV-era Soldier
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French Revolutionary Soldier
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Several DLCs dealing with French cavalry from different epochs
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same treatment of English, Dutch and all the other countries
You could also offer a
generic muslim soldier DLC for those who don't want to separately buy Ottoman, Mameluke, Persian, Mughal and Usbek sets.
4. Keep it simple with country flags and CoAs. From the box, England would just have a red flag, France a blue. The different flags can then be bought as DLCs.

5. Generic leaders, and monarchs, so no game is ever the same! Boring people can of course buy Gustav Adolph, Turenne, Magellan etc. DLCs. Or the "Spanish monarchs" DLC if they insist on having historical monarchs.