How easy, in general, would it be to make an easy mode for a game that only has normal? Is it workable?
How long is a piece of string?
There are any number of ways a game works, and any number of ways to make it easier by some degree.
For the basic approach you might not have to do anything at all other than slow the game down in an emulator (many allow for this similar to their turbo options) and give the user savestates.
Beyond that infinite lives/continues might be enough. Basic cheat territory here really.
Depending upon the game then removing a given mechanic (if there is a timer then freeze it, if there is mana/ammo then give infinite but leave health) can be enough and that can be done with cheats. For many systems you could even hardcode it into a game fairly easily, if not trivially on some later systems where they have tools to hardcode cheats.
You could add pickups to the level that work within the context of a game. In the case of streets of rage turn every pickup into a turkey and the game is far easier despite nothing else changing.
You could change the level itself to remove enemies, traps/pits, harder segments that might risk the timer getting low... or change any of those to slightly lesser versions.
You could change the controls if they are bad.
For an RPG you could edit the save to give your characters a few level boost, or put an item into it that does the same, or put end game pickups right at the start. Or you could double the experience gained to reduce grind -- while I don't consider grinding to be difficulty then some do, and now we have one of the bigger almost religious debates, probably only behind the liberal-literal-pizza cats debate from translation. I won't argue though that increasing or lessening grinding can change how the game feels to people.
For a game with some roguelike elements you could change things here (removing say permadeath from fire emblem).
Are some sprites/graphics hard to see in a given mode? Change them to be a clearer distinction -- we often see this with more modern FPS titles, and have fun and games when disabled people or colour blindness modes get added to account for it and then make something of a cheat, or modders doing cosmetic mods... but I can probably find an example of when it would be good in a 2d sprite based affair.
You could change the camera type to give you more warning ( https://docs.google.com/document/d/1iNSQIyNpVGHeak6isbP6AHdHD50gs8MNXF1GCf08efg/pub?embedded=true
), or otherwise code something to telegraph the upcoming hazards. Could be https://www.engadget.com/2016/04/11/punch-out-easter-egg-found-29-years-later/
, could be a sprite appearing in place on the side of the screen for a small time in the case of a scrolling beat em up.
Does a Luck stat have any effect on the game? Similar for critical hit, capture or whatever else calculations involving a measure of randomness. Pump it up. This could also come the other way and be a thing you tell the player about the algorithm ( https://www.dragonflycave.com/mechanics/gen-i-capturing
) rather than have it be "hidden". No hacking needed on their part if they know what the numbers will do, or don't have to piece it together for themselves.
You could change the movement or "AI" of an enemy. Of all the things mentioned thus far this is probably the hardest as far as what I would expect if I rocked up to an entirely new to me game tomorrow, especially for an older system.
Related to the above is hitboxes. Change these (either size or timing) and things can change in a game. Related to the "is or is not difficult" then what some call difficult others might call cheap, and if easiness is actually you wanting to reduce frustration with a game then this could go a long way. I would be worried about going too far here and making things boring but if you are the one making the hack then you are the one that chooses what to do here.
We also now have the reactions debate. Games frequently test reactions and rhythm. Patch out the code to instead just play like a normal game, or maybe just be more forgiving, and the game might change as a result.
Going on into the subjective realm now as well. I have a good memory and a thing for language and logic puzzles (probably not a coincidence that I get on well with ROM hacking) but others might not -- I make it something of a habit to read reviews from younger people when they look at older games and that is often what comes up. If you want to kick something like this in the head then maybe.
Colouring a bit more outside the lines, at least for around here. If you wanted to use a lua emulator to create a map on a game (or something like what http://www.romhacking.net/forum/index.php?topic=18717.0
does) then that might change how certain dungeon crawlers play (from having to memorise things like a proper maze, or make one on graph paper, to following a map in another window). Possibly subjective -- as long as you are not on a timer then there is very little difference in terms of information available to you, however if we go back to the frustration vs difficulty debate...
How difficult any one of those is... varies. I have had games where freezing a timer would break the game in various ways and following that I have to spend hours figuring out a workaround for that. Other times for controls then if I wanted a simple press to move 4 pixels rather than 5 it is easy enough, though if you had to learn some assembly first that might be harder. Graphics can be trivial to tweak, as can stat tables, and at the same time they can be buried deep in the game somewhere. Even savestates and slowdown probably won't help if people get hung up on logic puzzles, to say nothing of slowdown probably causing people to not learn a proper rhythm if it matters there.
I could possibly make some rules of thumb for an average game, where the coder did not get creative or try to block hacks, and maybe tweak it by "genre" a bit but it is not going to be any more useful than other rules of thumb, which is to say not very much at all in the long run.
Edit. Forgot co-op as well.
I like co-op games. Sometimes you can make one of these so the second character or whatever functions as an invincible helper (think Tails in the later megadrive Sonic titles). Can be good for the 5 year old sibling that wants to play approach so they can die all they like and not rinse a shared lives/continues counter but gives you something to do. Difficulty wise then probably easier to change an existing co-op game into this than add it into something that never had it, but at the same time could be harder than some simple add a second player sprite or give them controls over an on screen ally. Equally if you have to go back and tune enemy counts if the main player is doing the bulk of the work now then that can be a lot more involved.