At first, modding weapons and warframes seems easy – and for the most part, it is. Eventually, though, you’ll be drowning in mods, and a weapon you thought might become a beast is starting to feel a little underpowered. Given the mod system in place, this shouldn’t really happen – there really are very few weapons in game that can’t be made effective with the right mods (although only a select few are top tier, of course).
There are a few things to keep in mind. Firstly, you need to max the rank of your mods. Yes, this will increase the drain on capacity, but it’s absolutely necessary if you are chasing power. Initially, you should aim to max (via the Fusion process) important mods like Serration, Vitality, Redirection, Elemental mods, and so on, but eventually, you’ll want all mods you use to be maxed. Keep one copy of each mod unranked, and another (if you’re lucky enough to have three – some mods are quite rare) with only a couple of ranks. This will allow you to still use the same mods on partially levelled gear. Of course, maxing out certain mods is expensive, and I’m still yet to fully max several of the mods I’ve mentioned, but just keep it in mind!
Fusing mods to increase their rank costs credits and endo. Credits you can come across by doing missions – look for missions that used to be called Dark Sectors (there should be one mission on each planet that awards more credits than usual), or if you are further in the game, give The Index a shot (this is essentially a PvP mode against AI bots, played for credits and rare mods). Endo is harder come by – make sure you pick up every Ayatan Star you see (these are random drops from crates and lockers) and keep an eye out for Ayatan Sculptures. These are essentially trophies into which you affix Ayatan Stars. You can then sell these to Maroo (at Maroo’s Bazaar in Mars orbit) for endo.
Eventually, you are going to run out of mod capacity. At this point you need to make a decision – do I like this weapon/warframe? How MUCH do I like it? If you don’t like it, look for something else, but when you do find something you like, you have two options, both of which will be necessary to increase mod capacity. The first is in the form of Orokin Reactors/Catalysts (Reactors for weapons, Catalysts for warframes). You can only apply one to each item, but it will double the overall max capacity from 30 to 60. The issue is that these are very hard come by – there’s usually one Alert per week, but of course this may happen while you’re asleep. You could, of course, buy them with Platinum, but that could become expensive (they are 20P a pop). You might be lucky enough to get one as a daily reward, but they will be few and far between. This is why you need to think about how much you like it – if you don’t think you’ll still be using it at the end game, then maybe hold off for now… The other option we’ve discussed before – Forma. Applying Forma is totally necessary for the long term, but will be a timesink, as you’ll need to re-level the item. It’s worth it in the long run, but painful nonetheless. At the end of all this, you’ll be able to put (almost) whatever you want and turn the item into something unique.
At this point, the issue you will have will be in regards to the mods themselves. Common and uncommon mods are fine, but late game, they won’t be much help. You’ll need to find rare dual- and three-perk mods by grinding (this means a mod could have both increased damage and increased status effect, for example). Nightmare missions (which appear on a planet once you’ve unlocked all nodes) award dual-perk mods. Or you could go on a vault run on the derelict, and search for Corrupted mods – these mods are highly sought after, and essentially give a huge boost to one stat at the expense of another (huge increase to power strength at the expense of power efficiency, for example – so your abilities will be stronger, but cost more to use).
Lastly, you’ll want to search for Riven mods. Riven mods are random awards from sorties (more on that later) and from The War Within questline. These mods are specific to a weapon, and contain completely random stats. This could result in a supremely overpowered mod, or something somewhat useless. Riven mods come in different “dispositions” which essentially refers to the potential power level of a mod – weapons that are more popular have low disposition, so Riven mods are only moderately powerful, while less popular weapons have a higher disposition and can thus have a more powerful Riven. The issue with Rivens is that when they drop the stats are “veiled” – in order to unveil the Riven mod, you need to perform a specific task (such as stealth kill a certain number of enemies without setting off alarms, for example). Some of these are insanely difficult, some are easy – it’s entirely randomised. Note also that Riven mods are often Mastery locked – some of them can be locked up to MR16 (possibly beyond), so they are definitely leaning towards end game.
Once you have a good set of mods, and a few Riven mods, you should be fairly set with some weapons and frames that can get you through all of what the end game has to offer (note that you don’t need Riven mods to make end-game viable builds).