TL;DR
When loading a save with anything other than the current active set of mods, ask the player if they would like to sync mods before loading or just load with the current set.What?
Pretty straightforward: if the save file you are loading has any delta between its mods and the mods your active instance of factorio is running, just pop up a prompt mentioning this fact and asking "load and sync mods" or "load with current mods". There's already a sync mods button on the load screen, literally just prompt that functionality whenever it applies. Bonus if the window includes a list of differences, ideally even categorizing differences as simply a mod version vs. wholly different mod/mods.Why?
I actually swear this used to be a feature, but maybe it was a mod itself.If I'm loading a game to continue playing it, chances are I want to continue playing it as I was last time. Changing mods can break things (sometimes in not immediately obvious ways), disable achievements, stop replay recording, or even just mean having to go back to a previous save (if one exists) and redo things because you didn't realize you'd accidentally loaded a game with different inserters modded in because you were playing with them in creative mode.
Personally, I've been playing Space Age entirely vanilla (literally the first time I've ever done that since buying Factorio; I installed my first mod before even opening the base game), so why not go for some Steam achievements at the same time? I will still occasionally pop into a creative save to blueprint something and/or use Helmod. It's annoying that every time I want to go back to my main game I have to remember to hit the "sync and load" button instead of just being prompted with a confirmation dialog box.
Minor nuisance for me, but I don't see any reason loading a save shouldn't assume the player at least most of the time wants to pick up where they left off in as many ways as possible.