I have a few games that are still development that are facing this same problem. They want character specialization while maintaining choice. To ensure that this happens, they have the level cap. All of these games have the same problem where things get a little rough for the solo player. As a solo player, there are certain skills you need. From a solo survivor’s perspective, the more you can be a jack of all trades, the better your chances. Generation Zero has the added wrench in the works that your character could be solo character part of the time and teamed up another part of the time.
The common denominator between these games, and I believe the source of all the headache, is that they all have placed the basic survival skills that any player type would logically want into a tree or among many trees that a person may not want anything to do with.
It would be nice to have the basics pulled out of specialization trees. Maybe you get limited specialization points that stop at a certain level, but all the basics level up slowly with your character. In Generation Zero, some of these basics would be max health, max stamina, heal speed, stamina regen speed, run speed, aim speed, throw accuracy, and lockpicking. Of course, there needs to be a cap to this as well, but this is the part that should take many, many months of gameplay to really develop.
This doesn’t mean that a specialization cannot add onto these basic skills. For instance, maybe there is some new melee build and it has a perk to reduce stamina 2%, 5%, 10% when using melee weapons. Gain 2hp, 5hp, 10hp for each kill with a melee weapon. Etc. Add in the build specifics. 2%, 5% 10% more damage to enemies with melee weapons. These should probably be in this order going up the tree as well, or in a similar way such that the higher up the tree, the more beneficial for that particular class. Lower in the tree is more beneficial for anybody. Now apply this concept in similar ways to each build type.
Going the Marksman path, you currently get aim speed. If aim speed was a basic skill, then this would become aim speed increase when using rifles. If hip shot accuracy was a basic skill, then you could also get improved hip shot accuracy with rifles going this path. Apply to recoil and weapon sway, but only for rifles. This allows you to beef up similar abilities for trees that would use other weapon types.
The last specialization perk, besides added ability, could also unlock a unique craftable that cannot simply be randomly found in the world. Perhaps go as far as to make it so that only players with the specs can wield it. Specialization should be meaningful in the end if people are going to accept the caps.
Finally, XP gain increase shouldn’t be in any trees. It has no benefit in a capped system and does more harm to a player than good. However, if the basics were pulled out and leveled slowly with the player, it might make sense to place one of these at the end of every tree. For instance following the melee build as an example again, 1% 2% 3% XP gain for killing an enemy with a melee weapon.