I want to make all the gold rewards better for OOO standards (where you have 50.000 gold and you're only given 1000 gold for beating the grand champion...) it was easy to change thieves guild and arena because they are already set by script, but I remembered the dark brotherhood and FG rewards are leveled... I want to change that, would it be safe to change their scripts with the "player.additem x" just like the arena or the thieves guild in those?
Also, I'd like to make all leveled items be the best version when you get them, no level dependency (I'm not sure if OOO/FCOM already does that though), if that's overpowered I'd make the quest harder but that isn't a problem... would that be safe too?

Adding the line in every quest script to add additional gold is pretty safe. Making all the quest rewards be top level is also rather safe. Making the quests harder isn't safe at all.

Well, it depends, really. Are you going to use this mod for yourself only? In that case feel free to tweak whatever you wish. The problem with making quests more difficult includes modifying a lot of creatures, dungeons and places, which can cause plenty of conflict with the billions of billions mods already out there.

It will be safe in the sense that it will work as intended with no other effects. But it will also make your mod incompatible with any other mod that changes the same scripts for any reason, so that is something you must consider.

You could go the route of my Enhanced Economy mod though, where you have settings that will change the gold rewards for any quests. It is done not by altering the existing quests, but have a separate quest that runs all the time monitoring what's happening in your game, and then calling "Player.Additem Gold001 #" as it sees fit.

I use enhanced economy and it's amazing but there're some rewards which are fine, like say the arena rewards when you're grand champion, and if I made it x3 then that would be too much...

By making the quest harder I'd just change the level and maybe items of some NPC, for instance I changed Agarmir and his friend to be level 10... why would that conflict? And if it conflicts, what happends exacly? I'm running a mod made by myself which changes +100 vanilla named npc to wear more varied equipment and from FCOM, like Moderyn Oreyn wears armor and weapon he deserves, Lucien Lachance carries a very good dagger from fran and a very long etc... I love variety so I even changed guys with iron armor to some other armor from fcom with the same stats... I also added stuff to cells as rewards, added new bosses and notes, made horses stronger and much more, I call it OOO overhaul xD...

But now I wanted to do this reward thing, usually add it with the console (the gold) but that isn't inmersive

In a separate esp I got a mod which lets you buy spells to summon MMM creatures, but I did that one 5 months ago and only covered 3-5 I dont remember... in another ,esp changed vanilla and OOO bosses meshes with MMM more apropiate ones (example: Lich from Miscarcand looks like I Lich Lord) this one is mergable for some reason so it works well xD and in another one changed some names from generic NPC (mergable as well) so Ravagers are Raiders again and Vampires are like in OOO (ancient, deadly, fearsome, lesser) for some reason didn't like the name change : D

My game crashes sometimes, but it has always crashed so I don't think it's because of this... I'm playing close to the 255 mod limit and haven't had trouble completing quests from mods... (Kragendir's death quest runs fine)
But for some reason I can't import things from my mod to the bashed patch, therefore some NPC remain unchanged because I load it before bashed patch... what could I do to fix that?

I thought some people would like this mods but never uploaded them, before I'd have to fix some things like make the inventories or whatever importable so the NPC uses the stats and items in my mod xD

Well, I try to do the gold reward thing... and the items too, but not all of them will be unleveled, the dark brotherhood ones would be too overpowered xD

Why would it conflict? Imagine one character is changed to a vampire by one mod and another uses his as a redguard. Use both mods together and you won't get blade. This is a simple example which wouldn't cause real error as long as the mod with higher priority has his correct place in the load order. Stuff like changing inventories, quests, levels, that can cause some serious conflicts.

This is why most modders don't dare mod vanilla Oblivion unless they are 100% sure they're changes will be worth it and even then only if there are no mods that do that, or at least that number is extremely small.

OOO already changes the stats and items from a lot of vanilla NPC, that's basically what I did to make the game feel more realistic, I didn't change races at all just inventory and stats... I didn't remove their items though (thought something bad could happen xD) just added new ones.