The Tavern

Some species gaining XLs faster than others might sound interesting but let's look at what it really does.

The main beneficial effects of XL are:
- you get more max HP
- you get more max MP
- you get more spell slots
- you get more stats
- for some species, you get new/better mutations

There are already other aptitudes and mutations specifically for modifying max HP and MP, and another aptitude for modifying spell slots (Spellcasting aptitude). So these benefits of a high XL aptitude are unique; the main difference between XL apt and a slight modifier to these other apts is that the XL apt eventually becomes meaningless since almost every character wins at XL27. That's not really an interesting piece of differentiation.
The species-dependent effects of XL - stat gain and mutation gain - are already species-dependent. So XL aptitude doesn't introduce any additional contrast; there's not a demonspawn with a +1 XL apt to complement the demonspawn with -1 XL apt, nor should there be.

What really makes this aptitude an aggressively bad feature, in my opinion, is the main drawback of XL: with most gods, you gain less piety the higher your XL is (especially noticeable with okawaru), so xp apt acts as a modifier on your piety gain; Ds ends up with more piety than Hu, when an unspoiled player would expect the two species to get the same amount. This makes piety gain more difficult both to perceive and to balance, compared to every species having the same XL gain rate.

I suggest removing exp aptitude and making all species level up at the same rate as a -1 exp apt species would. Op, Te, Gr, Na, Hu, Ko, Ha, and Fo can get an HP/MP bump if they end up too weak after this, and Dg can have its HP modifier reduced to 0. While the species balance implications of this are significant, I think they're beneficial: every species would reach a given XL at approximately the same time, rather than having 4 different rates, so "what XL is a typical character when they reach Lair" becomes a much easier and more useful question to answer.
Furthermore, the effect on overall difficulty is very small. The best species (Tr, Sp, Ce, DD), plus Mi, all have -1 XL aptitudes already. Those games would be completely unaffected. On the other side of the power scale, DE and Mu are unaffected, though Na gets a little worse. The species that use XL extensively as part of their main gimmick are either only somewhat nerfed (Gr, Na) or completely unaffected (Ds, Dr). I believe the gain in clarity and simplicity is well worth the work.

(I'm aware XL has a bunch of other minor effects, like changing god wrath, Xom action breakpoints, affecting accuracy and AC in some forms, amount of skill xp you get from potions of experience, trap detection chance, etc. but I don't think those are important enough to matter here.)

Concrete aptitude changes I would suggest:

If we did want to make it actually relevant in the long run, the XL apt could also effect the *maximum* experience level you could get to. I'm not confident that's better than just removing it, but it's an alternative option, I suppose.

Level cap for each race would make balance hard to see. This race X gets stats quick but maxes out with 3 less levels compared to the other race Y.

Xp affinity means a trade off of early or later advantages but also means some species are terrible for extended. While others are boring early.

Since races doesn't have to be perfectly balanced against each other XP affinity is just a pasaive hassle rather than a good factor to influence race choice.

I've sent an email about this proposal to the c-r-d mailing list. No idea what'll come of it, but I think the suggestion is solid.