Future - I'd say no problem, except maybe Power Armor infantry ala Starship Troopers (but then from that book, each soldier would be an independent armored unit so even them are doable - sort of, no power jumps but so be it)...
Past - hard. You may have to change the game scale to be able to distinguish between meelee weapons (range 1 hex), pikes and other "long" weapons and ranged weapons. OTOH that won't be so big a problem as ranged weapons would have less range as well. However it would be hard to simulate things even so recent as Napoleonic compact column charge - let alone Hoplite phalanx, using the inertia of tightly packed men against enemy formation. Also Carrees would be hard to form
I'd say historical limit (if pushing it already) for the game engine and current scale would be period when riflemen and artillery were already major part of the battlefield. Go to swords, halberds, pikes etc. and you open an entirely new can of worms.
As for changing the code, better not think about it, as a) only Don and Andy are permitted to meddle with it and b) any day I spend at work, testing generally simple software, I'm getting shivers from thinking what a work is it to twist the C spaghetti of SP2 engine into such things as reverse or barbed wire
So... If you want, you may create ancient OOB set but don't expect any support in form of code meddling etc. Don and Andy have enough work with SP as it is...
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Seems I'm beginning to spot first changes of my mind already forming up to future PhD studies (incl. leading lessons), sorry for the length of the post