Well I guess that this wouldn't work if you're only allowed to carry one weapon and one armor. If you can expand that, then here's what I was thinking:
When you start the game, you have an equally low level in all the game's skills. Sticking with Silver's idea of knight, marksman, and sorcerer, lets say you start off with a basic sword, bow, and fireball spell. Then you go off and do quests, ect. You use your sword most of the time, your magic some of the time, and never use your bow (it counts as "using" a skill when you kill a monster with it). On level up, the amount of skill points that it is possible to distribute to each skill (swordsmanship, archery, magic) reflects how much use used that skill. For example, in this case you might have 5 points available to put into swordsmanship, 3 points to put into magic, and nothing to put into archery. Of course, the total amount of of points you can distribute is limited (let's say that you're allowed to distribute 4 points in total). So you need to think about how you want your character to progress - if you want to focus on swordsmanship, put all four points into it, if you want to focus on magic put 3 there and one in swordsmanship.
So this forces you to think about how you want your character to develop, as if you want to be good at two skills you need to use both of them actively. When you get out of the starting area and starting delving into tougher dungeons, you really need to be strategic. You might want to get better at magic, but your magic isn't strong enough to take out the demons you're battling, so you have to use your swordsmanship to weaken them and then your magic to finish them off, and then gain points to add your your magic skill. Â