In terms of advancing the tech level, I've been messing around with some 1800s/Napoleonic-era rules modifications, and to be honest it's very difficult mechanically.
The big hurdle is that the fundamental guts of the combat system in D&D is very melee-focused, and it operates under the assumption that armour is effective and useful. The later the time period, and the better the firearms, both of these become less the case.
Basically, Strength becomes pretty much useless, and as armour also becomes obsolete and ranged combat increasingly dominates, Dexterity becomes an uber-stat. I've messed around with assigning firearms a Str prerequisite that a PC must meet in order to use them without suffering to-hit penalties from the recoil, but this is really just tinkering around the edges. The AC issue is one that I haven't thought of a way around. But without some sort of tweaks there, non-spellcaster PCs would all be a bit samey with high Dex and Str as a dump stat. Except barbarians might do ok, but to be honest I haven't had much experience with the class, and how well any melee-only PC would do in a setting where ranged combat was the base assumption is another question...