So, here are the three most important keys for beginning:

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(1) ALT+R = This changes the resolution of the game, making the game look much better. Try it, you'll see what I mean. You'll want to press this command once you've selected/loaded into a level - pressing it at the main menu or overworld select screens, I'm not sure if it'll do anything.

(However, this may require you to utilize the following two keybinds to make it most optimal - it seems the default pixelated game runs fine at launch, but Alt+R makes the game run slower due to how Dosbox emulates/code).

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(2) CTRL+F12 = Speed up game. Might need to hit this a decent number of times to get it feeling like it won't be chugging/lagging. For me, 8-10 uses of this command makes the game feel as though its running as it should, but this may vary due to dosbox/machine.

(3) CTRL+F11 = Speed down game, incase you set it too fast with the above command. You can play around with the speed and figure out what feels good; I'll have to sit down and figure out the optimal number of speed ups to make it run right.


Courtesy of @-= SilverMike =- :
"CTRL+F11 and CTRL+F12 are for changing the Dosbox cycles. You can change the frameskip using CTRL+Numpad+ and CTRL+Numpad-. Note that, whatever amount of high cycles you're using, the game will always try to cap the framerate of the game at 20 game turns per second, excepted in the menus."
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NOTE: DK1 runs via Dbox emulation, which utilizes your computer speed. I can't say exactly how it works, only that the following number of speed up commands may be more, or less, depending on your machine - but it should be a good base point to figure out from.

Short version: There's a compilation of fixes with the ability for additional content (ie: Steam Workshop esque) on the side called Keeper FX. It fixes some of the hotkeys and in-game functions that seem to have stopped working properly (ie: picking up creatures via panel, highest level/lowest level/etc).

Located here:
https://keeperfx.net/

It says commentary about new creatures, but as far as I'm aware, they're only on custom maps/non-original campaigns - I haven't seen anything during my current playing of the normal game. "New creatures" is listed as a bullet point more for the fact that it CAN be done now, something which wasn't necessarily possible without replacing existing game files in the past.

That said, if I do encounter anything new during playthroughs of non-custom content (campaign, skirmish maps), I'll definitely come back and update this part of the guide.