Yea I just described how EVE does it for Tad. Everyone having access to all skills makes sense in their world.
For the fantasy version I'd want there should be 'soft' classes that have specific skills to learn that the other classes dont get, based on character creation choices. GW1 kinda did that with their dual classing. Each class had 4 attributes that their skills were based on, one of those was the 'main' attribute. When you picked your second class to complete the character, you didnt get the main attribute but only the three others.
Same with races although I would put most of the emphasis of race choice into the faction system (as EQ did).
Another difference to EVE would be how you gain and improve your skills. Instead of buying all skills like in EVE, you add them to your repertoire through different ways. Learning he basics at a school or academy in town, discover them in old tomes in the dungeon, granted by your deity, taught by the old grandmaster after you complete his task, etc. Getting your skills so you can train them up is a big part of the gameplay.
Meanwhile you store up the xp you get for playing, and can assign it to the skills you have acquired to improve them in the same way EVE does, just without the time restriction. That means people that play more can catch up to casual old timers, but still only gain more choices instead of direct power. The game should probably work on a limited available action set like EQ or GW2 (limited abilities, on the fly switching difficult but possible depending on circumstances) or GW1/EVE (you go in with ready skills and thats it until you return to a station or outpost).
Re: Hybrid tax:
If you allow entirely free use of all skills, you need some checks to balance that. EVE allows you to learn them all but depending on ship choice you cant bring them all to bear. IF for example your game allows a character that has learned skills from the mage and warrior and priest archetypes to use them all at the same time, you might have a problem.
You can still solve that by specialization though. Again, an EVE example: skill has 5 levels each giving 5% to small autocannon range. Level 1 takes 1 days of xp to fill up, lvl2 3 days, lvl3 9, lvl4 27 days, lvl5 108 days, lvl6 ... you get the idea (dont argue the numbers, just illustrating). Now in EVE lvl5 in a skill is the end and everyone can max it because the restriction to not using all skills comes from the limited equipment choices on your ship. There are also several skills that improve said small autocannon (range, target tracking, fire rate, etc etc). Given that the last level of each of those skills tages ages compared to the first 4, you are actually fine for combat with lvl4 in all but the most important skills, too. However it does give you a very longterm goal to get those last few percent for omptimal performance on your most-used ships.
Back to your classless Human in a fantasy setting. Since you dont have the restrictions of ship layout to stop him from throwing fireballs while healing and swinging a polearm, you can encourage people to specialize by having enough different skills that affect polearm combat that also getting excellent healing and fireballing is spreading the chacters proficiency in each of them too thin. People are encouraged to focus on their main activities to advance the character in an efficient way, but (like in EVE) they are allowed to fuck it up by being paint sniffers and learning 1,000 skills to level 2. Thats ok because you cannot 'ruin' the character. At best, you enhance your character in a way that you enjoy and that sets you apart from all the other warriors by having another trick in some situations, at the cost of being that 1% less dps in general. At worst, you invested xp(time) in something that doesnt work out, then you have some skills you barely if ever use. You can actually expect most people to do that because trying out the skills doesnt cost much, only maxing them. And the game being about sidegrades shouldnt punish that harshly.
The only issue I see is if the game runs very long and people actually have the time to max polearms (3 years), medium armor (3 years) and cooking (7 years) and then decide to go back to those old druid skills they tried out and forgot about to max those (5 years). So 18 years later you have that OP polearm druid that throws delicious cake at you. The easy solution would be not going classless but with soft classes ala GW1 (see above), but I'm sure if a dev wanted classless they could figure out a way to stop that druid from becoming a problem before the 18 years are up.
