Every game (tennis, golf, tiddlywinks, everything) has rules that restrict the means that players are allowed to employ in achieving the goal of the game. Players like those rules because they make the game more challenging. So how do we know that making it harder for players to maintain their desired character build won't attract more players to the game? I don't think it is wise to make assumptions about what other people want.
I don't think I'm making assumptions because every single learning by doing system with a skill sum cap I can think of gives people both options and they overwhelmingly pick pinning their skills over perpetual drift. I think there is no reason to assume that they would be happier if their preferred option was removed.
I prefer systems in which every character (even starting characters) can do everything - just not necessarily very well. Learning is the process of gradually improving existing abilities, rather than suddenly acquiring previously absent abilities.
I'm with you on this one, I like systems where characters aren't arbitrarily unable to perform basic tasks. I would like it if characters could maintain a certain level in almost all skills, and then there was a layer of ability above that that encompassed your "edge" in that skill, which you could only maintain for a small number of skills.
I think there should also be some character traits that are mutually exclusive. Especially when it comes to magic I find it kind of makes characters less interesting if everyone can do magic and there is nothing you have to give up for it. It takes away the ability to play a mundane character.
Outward did a pretty good job of letting everyone use everything and learn the rudiments of every skill tree, but keeping builds tight. It also made you sacrifice health and stamina to get mana, so there was a reason not to learn magic (Though even just having a tiny amount of magic made the game a lot easier).
I have likes and dislikes for each type. I like the freedom of being able to do anything, but I dislike feeling like a number instead of a unique character. I like being unique and bringing specific skills to my class, but I dislike the lack of freedom of choice these systems bring. I like the freedom to change between different roles, but it makes the character in my head more muddled, who am I?
I think there needs to be a good mix of skills that everyone can learn and character traits that you can only obtain by making a hard choice between different ones. The really big things that I don't think everyone should have are the things that are essentially superpowers. Magic for one simply isn't interesting if it doesn't require any serious sacrifices and simply becomes a handwave for anything anyone does that makes sense as a game mechanic but not as a story element.
I like it when characters have some significant abilities that are exclusive to others. Especially when it comes to archetypal stuff. In a game where you're supposed to be able to change your character around these could be something like which god you follow determining how your character's magical potential is utilized.
For example, you'd have four gods, one for each general archetype.
The Warrior - Favors self reliance and prowess at arms. Followers of the warrior don't do magic, instead they use their mana as a kind of shield that absorbs a portion of incoming damage.
The Trickster - The trickster's followers don't exactly do magic, but have the uncanny ability to go unnoticed in plain sight. The mana bar is spent to be invisible.
The Healer - Followers of the healer are unmatched at providing aid to their allies. They can learn magic that heals people.
The Sage - God to those who seek deep knowledge and use it to control and confound their enemies. Followers of the sage learn magic that affects the mind.
So within that framework you can then leave people free to pick whatever skills they want. There is a reason to have stats that give you more mana even if your character doesn't know magic because your deities gifts always run on mana. You could learn any weapon group, tactics and leadership, laying traps and fieldcraft etc and basically create a character that is a combination of a general fantasy archetype and a bunch of skills you'd see in more historic fiction. You'd be able to get a lot of different character builds out of it without having to make things like "Turn invisible" or "Restore health" into simple skill trees where everyone will always pick up the basic level because having even just the most basic competence in those things makes any character much more powerful. Instead these would be skill trees you can only acquire by choosing it instead of the others.
By having it be related to a deity you worship you also still have the idea that you can switch between these. Maybe switching faiths means having to start training the associated progression from the ground up again, because it takes a level of devotion to gain these benefits, so switching rapidly all the time isn't a good idea, but you're not locked into a class choice either.