Gimme Diminishing Returrns

I want to play a game of DnD where a bunch of level 1 character can fight alongside a level 5 character and still contribute.  I don't want PCs to start out being badasses, so I guess it'll have to be a low-level, gritty system.

There's some precedent for this.  Even in 0e, saving throws eventually slowed their rate of improvement, and +1d6 HP per level eventually becomes +2 hp per level, which I suppose is slightly less.  But by then, the PCs have already killed kings and tea-bagged tyrannosaurs.  I want a small, powerful little system that plateaus early.

For this to work, HP, skills, and attack bonuses cannot advance linearly.  I need a way for skills to be reined in heavily beyond third (?) level, but still easy to obtain at level 1 and 2 (because players like their level  1 thief to be mechanically differentiated than the other level 1 thief in the party).

I can think of two ways to do this.



One option is to mandate that each successive rank you take in a skill is narrower then the previous.  I've seen this before.  So you might have Stealth +1 and then take Woodland Stealth +2 next level.  This is good because it is simple, but its also self-limiting, and I a lot of skills don't break down into a lot of subcategories.  What exactly is a subcategory of a subcategory of Swim?  Is swimming in a storm-tossed sea of urine so much different than swimming in a giant bowl of chicken soup?

The other option is to have a point buy system.  So Swim +1 costs 2 points but Swim +2 costs 6 points.  You get 10 points per level to spend.  And then Swim +6 costs 100 points or something.  This is probably closer to how skills are trained in real life.  And it has the advantage of encouraging characters to generalize, and it keeps the character sheets simple.  The downside is that it's a little clunky, and probably requires you to consult a chart.

And here's the most interesting idea I've had all day:

What if base attack bonus was purchased with a diminishing return point buy system (like the skills)?  Fighters could get a discount or something.  This could even let you merge Fighters and Thieves into the same class.  Some could specialize in crackin' skulls and punching babies, while the others could learn to hide their toes show when they hide behind a tapestry and how to hold in their farts real good when they're thiefin'.

Are there any games that do this?  I'm not creative enough to have an original idea (and I not very familiar with the world beyond the different DnD incarnations).  Who does this?