Abilities Acquired Diegetically

I have been thinking about character abilities lately; more specifically, about how they are acquired. In OD&D and B/X (and most games derived from them) characters do not gain new abilities as they grow in levels (except for new spells); they only improve their already existing ones. In AD&D (and its derivations) character do gain new abilities, but they are tied to certain levels. In newer editions of the game players may often choose their characters abilities (be they feats or class abilities) off a list.

I myself do not find character building much entertaining, but mostly because it is cumbersome and it is removed from the game world (i.e. my choices as an adventurer have no impact on which abilities I might be able to take). On the other hand, it feels rewarding to being able to pick a new ability and using it as soon as the right circumstances arise.

The solution might be making almost every character ability diegetic regarding their acquirement. That is, classes could be designed with only a few key differences (permitted weapon and armour; advancement of to-hit bonuses; hit dice type; and one or two distinctive feature), and we could provide ways of gaining new abilities tied to the setting. To keep niche protection, abilities could also be tied to certain classes (that is, a character may not acquire every available ability). Also, if abilities are not equal in power, the difficulty of their acquirement can be adjusted properly.

How many hits do I need to kill that thing?

I was trying figure out the minute (or not so minute) differences between various hit point and damage systems. For that I just made a simple spreadsheet (available here) where I calculated a number of things - I thought it may be some help to others.

I provided a minimal amount of notes (how I obtained the average HP and damage at a given level), but the following should be notes, as well. The first and sheet includes how many HP a character (or creature) may have at a given level and how much damage he may deal. Assuming an opponent of equal level or Hit Dice, you can find the number of hits it may take on average; after that, how many attacking attempts are required depending on your chance to hit (25%, 50%, and 75% of success, respectively).

The third and fourth sheets differ: they show you how a character of a pre-defined level (1st, 3rd, 5th, and 7th) compares to creatures of various Hit Dice. Therefore, average HP is calculated for the creature and damage for the character, and the number of hits or attacks are calculated from the character's point of view.

2014. 03. 13. edit: Last night I updated the file for there were some incorrect functions. Shouldn't have done it so late, I guess.