I have three things in mind regarding Oblivion XP:
1) Is there a plan for even more actions gaining xp? Trading with merchants come in mind for instance.
No, not really. I will be tweaking how some of the existing actions generate experience points. For example, right now you get XP for picking up a stolen item - I would like to change that so that you get XP for fencing a stolen item instead. I would also like to actually implement getting XP for changing an NPC's disposition. Currently there is a place-holder for it in the ini file, but SirFrederik didn't actually write the code to implement it, so I guess that's one item that will be added, if possible.
2) How many XP is there (roughly) in vanilla Oblivion? I ask to know what is realistically the maximum level one could gain if he went through all the content NOT counting the respawning. If the total nets say 20 levels with default settings then to increase in level you would either need additional content or do one of the tedious things: wait for dungeons to respawn and go killing OR do infinite xp gains stuff (like pickpocketing). Neither is fun or particularly appealing imho.
3) Related to previous question. If one embraced similar philosophy to character development as in other popular levelling mods like Realistic leveling or nGCD then reaching final level (cap) should also mean reaching 100 in all skills and attributes. (I am not counting with uncapping here) The amount of points per level would then be:
attribute points: (800 - 'sum of starting attributes') / 'level cap' [i.e. 800 - 355 (in my case using RBP) = 445 / 50 (desired level cap) = 9 (8,9)]
skill points (skillPointPreset = 0): (2100 - 'sum of starting skills') * 3 / 'level cap' [i.e. 2100 - 325 = 1775 * 3 / 50 (desired level cap) = 107 (106,5)]
Note: I believe that skillPointPreset = 1 should affect only how one progresses through levels and NOT the final figures (maybe I am wrong though).
That's why I would like to know what is the real level cap when using Oblivion XP sou I could set things appropriately if I wanted to be able to reach it by finishing (mostly) what the game has to offer and reach the caps for my character's statistics by then as well.
Thank you!!
I actually don't know! I have adjusted the number of skill points per level that I have to spend to line up better with efficient levelling. I also have the skillPointPreset set to zero because I usually play a "jack-of-all-trades" type character, so I increased the skill points per level to 105. In addition I slowed my rate of levelling by increasing the Quadratic Slope to 1.2. Even with these settings, my character has maxed out his attributes by level 32 - you can see my character's stats on the images page in the Nexus download. Had I left the rate of levelling at the normal levels, he would have levelled even faster. I don't have a lot of quest mods, but I did have The Lost Spires and A Brotherhood Renewed loaded. I have recently loaded Enhanced Economy and activated the quests that are available in there. And I also purchased Shivering Isles and did a couple of quests in there, but haven't completed that quest line (and probably won't

). I only purchased SI to support Oblivion XP

The extra quests from EE and SI have allowed my character to advance a few more levels, but I believe he would have maxed out his attributes even without these extra quests.
From what I understand of SirFrederik's intentions, he developed Ob XP with specialist characters in mind. He also developed it so that you wouldn't max out all your skills playing a regular game. However, Ob XP is so customizable that it is easily possible to max out your skills and attributes if you wish. Having 13 attribute points to spend each level is actually more than what you would typically have with vanilla levelling because with Ob XP, you can actually spend all 13 on your attributes. With efficient levelling, it is hard to get those kinds of increases
every level. Also, if you want to increase your luck, you can only spend one point on it. With Ob XP, you can spend as many points as you want on luck. And the number of points you have to spend on your attributes is also configurable.
The skillPointPreset allows specialist players to get by with less skill points per level. So if your major skills are also your specialist skills, then increasing them only costs 2 points per level. This allows you to increase your major and specialized skills by a total of 18 each level with the default settings. To me, even this wouldn't be enough and I would increase the number of skill points per level. However a lot of players like these settings and I know of a number of players that actually decrease the number of attribute points they have to spend each level. I think there are a lot of players who want a challenge throughout their entire game and find that the game gets too easy once they start to max out their skills and attributes.
I'm not sure I've answered your questions, but Oblivion XP is extremely customizable. I suggest leaving the number of attributes you have to spend at 13 per level and increasing the number of skill points you have to spend. I basically figure out how many skill points I would need to increase my attributes by 13 points if I was doing efficient levelling under the vanilla system. I demonstrate how to do these calculations in the readme file that is included in the download. I think that if you do that, you will be able to max out your skills and attributes somewhere between levels 20 and 30 without having to revisit dungeons. I don't redo dungeons either and don't even clear out half of them when I play - I hate dungeon crawls if there is no purpose behind them