So one of my long term gripes about the series is that radiation just doesn't seem to be a prevalent threat in the various wastes. Maybe if you are a dirt poor settler that can't buy radaway, but as far as the player is concerned, radiation is an annoyance. Part of the problem is that radaway seems to be pretty common, and other factors thanks to settlements clean water is practically commonplace among other things. In FO4 I actually like the maximum HP reduction, as it fixes the: "Hit 200 rads, better pop some medicine." Thing. I personally think the overall problem is that radiation damage is too easy to fix, though making radiation damage super hard to fix by itself would be a bland approach to fixing this. Instead I was thinking of a tiered radiation system, where you have normal radiation damage, but additional layers that build up over exposure/neglect. While the fallout universe hasn't striven for realism, I think it would make sense if high amounts of radiation damage can't be completely fixed without professional aid and get harder to fix overtime if left to fester.

*Keep in mind numbers are just for examples until (if) I can make this mod myself to test thing out, the numbers are hypothetical.

Base Radiation Poisoning:

Self explanatory, the regular radiation damage that's in game, the only difference is that with this idea is that normal radiation poisoning will gradually transform into advanced radiation poisoning if once over a certain threshold.

Advanced Radiation Poisoning:

This type of radiation poisoning is more severe than normal radiation, and thus can only be fixed by qualified doctors (I.E named NPC's or lvl three clinics), and would show up as a dark red layer on your health bar. Kinda like this:

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For example say 1 out of 10 rads you take adds a point of advanced radiation. You jump into a pool of nasty water that has a rad rate of 10/sec for 20 seconds with no protection. You will accumulate a total of 200 rads, or 1/5th your heath. 20 Of those rads are advanced poisoning. If you were to ignore your poisoning, more of the regular points of radiation will become advanced if you put off removing them. Obviously, to keep players from constantly having to keep their health perfect, the advanced radiation poisoning will only begin to fester at say, 150 rads. Also, the worse your radiation poisoning, the faster advanced poisoning accumulates. So if you are at 500 rads, you would want to repair that as quickly as possible. Now, you pop a radaway, the 180 normal rads will go away, and the rest have to be fixed by a doctor. And considering how easy it is to make money in this game, advanced rad poisoning won't be cheap. Say, 3 caps per point of advanced poisoning. So to fix those 20 stubborn rads, that would be 60 caps.

(Optional) Permanent Radiation poisoning:

Some, small, masochistic part of me has always wondered if some radiation damage was so severe, it could not be repaired, and you have to limit your exposure as much as possible, lest it whittle you down, these would show up as a grey bar on your HP. Since this permanent, the chance for these would be far smaller and harder to get; say 1 in 200 rads accumulated are perma rads, and cannot be accumulated if you have a rad intake of <1 (so as long as you have adequate protection, they can be avoided) and does not fester.

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As far as modding this myself, I'm a rookie Newbie modder at best, and not sure how hard this would be to implement as it's a game changing feature. I've played around with the GECK for New Vegas, but I have a feeling that it would take the nuts and bolts code writing modding to achieve an overhaul like this, as I really don't know if the FO4 GECK would allow for something like this. Still, I wanted to share the idea you guys to see if it has merit.