@DragonParadox any hopes this will be a solo PC D&D game ?, there is a lot of freedom to be had from how different such a game would be from for example A Sword Without a Hilt , like for example you don't need to deal with keeping track of a large part and its easier to set up challenging encounters while encouraging creative solutions , making the players over powered is much less dangerous since its still one PC so no matter what the action economy is always against him so letting the player mess around with the more powerful stuff like artifacts and the like is a lot less likely to break the game and if the PC turns out overpowered you can just up the ante and put the solo PC against challenges that would normally require an entire party to over come and watch the fire works
there are also some pretty intersting ideas and mechanics you could introduce that in a game with more than one PC would get unwieldy and cause bloat , to give an example instead of having leveling up a case you gained X exp and your now level Y instead you show the process of the character advancing his class by studying , practicing and attaining great feats based on player choice and actions
to give a detailed example , your a level one arcanist and you want to become a level two arcanist then you need to do the following actions
1) study and practice (0/2) , this represents the skill points the players earn when they level up like knowledge arcana ,world , perception ..ect , along with the number of action points you need to put in to bring it to the next level , this Action point cost grows exponentially , going from level two to level three would require 4 AP and levels three to four would need 8 AP so on and so forth , this is both to prevent players from the rushing levels and to show why level 20 is considered a cap not hard cap mind you can totally go past it but it will be really Really hard as it should be , the PC could go to or hire trainers and teachers to reduce the AP costs but the higher the level the rarer the teacher and more expensive not in terms of gold and no such aids when you hit level 20 as that is the finish line for conventional skill and its more or less you pushing the boundaries of what can and can't be done at that point
2) research and experiment (0/2) ,this represents the class related specialized practices and traditions(like arcane exploits in this case) and how you raise your class's caster level as to allow for higher level spells to be cast , as an arcanist you accomplish this by studying magical theory , experimenting and reverse engineering the inner working and mechanics of magical phenomena (a spell and its effects for example) , this also increases exponentially but their are ways to mitigate that like for example arcane tomes and research journals you can acquire from libraries and magical book stores at increasingly high costs of gold based on your level to reduce the AP cost by as much as 75% since your not being forced to reinvent the wheel but that only applies for levels 1 through 10 , from levels 11 to 15 the books you need can only be found in the private collections of archmages requiring whole quest lines to get you hands on them and from levels 15 to 20 you reached the point were the knowledge you seek can only be found in ancient and deadly ruins , the lairs of elder dragons ,the hands elemental Sultans and domains of demon/empyreal lords , there are no aids to reduce AP for getting past level 20 at this point your at the end of conventional knowledge/understanding and are effectively trail blazing
3) accomplishing a feat , this is pretty self explanatory , this is how PCs gain feats , the players invest a bunch of AP to carry out actions relating to attaining a feat and this were some the fun of a Solo PC could come in for depending on how the many AP invested and the player write ins you could alter how a feat works narratively or mechanically (with GM permission goes without saying) allowing for some truly fascinating narratively driven creativity fueled munchkinry
4) lastly the last requirement the arcanist needs to level up is to train his mind and body through intense mediation to channel and control the new levels of magical energies he now understands how to wield followed by creating a partition in his mind to hold spells (getting a new spell slot)
lastly notice how I never mentioned at any point the PC being granted new spells just for leveling up , that is cause I prefer the Baulder's Gate method were if you want a new spell you go look for it , most first to fourth levels spells are easy enough to get from any magic store , fifth and sixth levels spells are harder only being found in the private collections of some mages or arcane academies with many of them requiring some good old fashioned dungeon delving to get your hands on them and for 7th ,8th and 9th levels spells due being supremely rare and down right mythical in some circles its either you go on massive involved quests for just finding a single one of those game breaking things or invent your own through AP hell
so what do you think ?