One solution would be to have different skills require different amounts of work to get to 100% proficiency.

This of course puts a limit on how far certain skills can be developed. How committed are you to the idea that a skill must be maxed out before it unlocks a merged skill? You could already enable certain skill mergers on earlier proficiency. For example, a character would already gain the option to learn Sticky Fireball with Fireball 30% and Stick Ball 40%.

If you do want to stay committed to the idea that mergers may only happen when a skill is maxed out, want mergers to happen early but also want to give the option to invest a lot of effort into mastering the art of fireballing, then you could solve that by also unlocking a more advanced version of a skill when it's maxed out. For example, when the character reached 100% proficiency in Fireball, they don't just unlock the merger with Stick Ball, but also unlock a new skill "Greater Fireball". Greater Fireball is basically just a stronger version of Fireball which also requires a lot more effort to bring to 100% proficiency.

One solution would be to have different skills require different amounts of work to get to 100% proficiency. When some skills max out very quickly, then it won't take very long until the first merger options become available.

This of course puts a limit on how far certain skills can be developed. How committed are you to the idea that a skill must be maxed out before it unlocks a merged skill? You could already enable certain skill mergers on earlier proficiency. For example, a character would already gain the option to learn Sticky Fireball with Fireball 30% and Stick Ball 40%.

If you do want to stay committed to the idea that mergers may only happen when a skill is maxed out, want mergers to happen early but also want to give the option to invest a lot of effort into mastering the art of fireballing, then you could solve that by also unlocking a more advanced version of a skill when it's maxed out. For example, when the character reached 100% proficiency in Fireball, they don't just unlock the merger with Stick Ball, but also unlock a new skill "Greater Fireball". Greater Fireball is basically just a stronger version of Fireball which also requires a lot more effort to bring to 100% proficiency.

That would mean that raising a skill to 100% gives two different progression options: diversification (by investing in a merged skill) or specialization (by investing in an advanced version of the same skill).

One solution would be to have different maximums for different skills. For example, if Fireball were already maxed out after investing 3 points and Sticky Ball after investing 4 points, then Sticky Fire Ball would become obtainable on level 2.

This of course puts a limit on how far certain skills can be developed. How committed are you to the idea that a skill must be maxed out before it unlocks a merged skill? You could already enable certain skill mergers on earlier levels. So a character with Fireball 3 or Stick Ball 4 would have the options to invest into Sticky Fireball 1, Fireball 4 or Sticky Ball 5.

If you do want to stay committed to the idea that mergers may only happen when a skill is maxed out, want mergers to happen on low levels but also want to give the option to level fireballing to absurdly high levels, then you could solve that by maxing out skills early, but also unlock a more advanced version of a skill when it's maxed out. For example, you could max out Fireball on level 3, and then not just unlock the Stick Fireball, but also unlock a separate skill "Greater Fireball", which is basically just a stronger version of Fireball but mechanically counts as a separate skill.

One solution would be to have different skills require different amounts of work to get to 100% proficiency.

This of course puts a limit on how far certain skills can be developed. How committed are you to the idea that a skill must be maxed out before it unlocks a merged skill? You could already enable certain skill mergers on earlier proficiency. For example, a character would already gain the option to learn Sticky Fireball with Fireball 30% and Stick Ball 40%.

If you do want to stay committed to the idea that mergers may only happen when a skill is maxed out, want mergers to happen early but also want to give the option to invest a lot of effort into mastering the art of fireballing, then you could solve that by also unlocking a more advanced version of a skill when it's maxed out. For example, when the character reached 100% proficiency in Fireball, they don't just unlock the merger with Stick Ball, but also unlock a new skill "Greater Fireball". Greater Fireball is basically just a stronger version of Fireball which also requires a lot more effort to bring to 100% proficiency.