okok i have little experience w vrmmo rps, BUT i do run a site w/ a magic system that works pretty similarly to what ur describing with the "strict" style, and i do think it has some advantages over freeform as u describe it.

[for context in the site there are x fixed magic types, and you select one and progress through the spell trees/skill trees. staff regularly updates with new spells that members can unlock for their characters]

pros of doing it this way, that i've seen (other than what u already have):

- control over balancing. u never have to worry abt approving sth that might be "OP" or not well-received, and it gets people off the ground faster.
- i've found it creates a fun community aspect to it—members will talk abt new spells, share in the progression system, it adds to the shared culture of the site in a fun way!
- the IC implications for lore are soo much fun with a strict system, imo. it makes ur world cohesive when the magic/ability system is set, and creates for interesting dynamics ICly compared to when each character may be coming from their own baseline system of abilities.

cons:
- it's a lot of work, esp if u intend to continue to add new skills as time goes on.

and about "freeform" — i'd still consider the strict version of doing things freeform because IMO, you can still interpret the skills and use them creatively when there are no stats/hard and fast rules. for example, consider a freeform pokemon site where all the abilities would be pokemon moves...there's still an element of creativity to stringing the movesets together, using ur pokemon to achieve certain goals, and mixing and matching effects! sometimes the abilities being set in stone helps pushes ppl to channel creativity elsewhere, so i kind of see it as a win-win!!

i would also say that it helps if there is enough variety in the skills u are selecting from so that members/characters can still feel like they are achieving unique progression (and not that all characters of a class tend towards the same skills or same niche)

also u could consider a hybrid thing where maybe at a certain point a character can learn a cool signature ability or sth, to add even more layers to progression....

goodluck w the system-building!!