If you challenge yourself and place restrictions on the tools that are available to you, then by necessity, you will learn how to use the tools that are available to you very well. You'll learn new skills as a player (like how to get the most out of spells, wands, and potions) and develop new combat strategies that suit your party. That's one of the reasons why I like to try new characters and parties.
I once played a party consisting of a Cleric/Ranger and Imoen all the way through BG1, BG2:SoA, and BG2:ToB. When you have only two characters in your party, you have to get the most out of them. Sometimes, when you have a larger party, one or two of your party members might do most of the work while the rest just stand around or provide back-up. If you have only two characters in your party, you'll keep both of them pretty busy. That's one advantage. Another advantage is that they will advance more rapidly than the characters in a six-member party, so you will have earlier access to a lot of cool spells and stuff. I removed the level cap so that there was no restriction on their advancement. My Cleric/Ranger were very powerful and very fun to play. Between the two of them, they had all of skills I needed to win the game with flying colors...at least, until I got to Throne of Bhaal.
When I played Throne of Bhaal, I installed the Ascension MOD, and I found out that I needed an extra Fighter in my party, so I restarted the game and added one to my party. Everything turned out great.
Here's another bit of advice. Don't be afraid to use some of the NPCs on a temporary basis. For example, if you need extra help on one particular quest, you can recruit one or two extra characters to help you and then dismiss them when you don't need their help anymore. When you think about it, there's nothing unrealistic about that at all. When Imoen dual-classed to a Mage in BG1, I recruited Coran when I needed a Thief's skills, but once the quest was over, I didn't feel compelled to keep him in the party for the rest of the game. In Shadows of Amn, when Imoen was taken away, I soloed most of Chapters Two and Three, but when I needed help, I recruited Keldorn and/or Nalia. My character went to Spellhold by herself and recovered Imoen, and for the rest of Shadows of Amn, it was just the two of them until I got to Throne of Bhaal. Then I pitted my "Three" against "The Five", and it was one of the best roleplaying experiences I've ever had.
No matter which character class you choose, you can't go wrong. So just try to think about what you would enjoy. If you want to get the most out of spellcasting for a change, then play a spell-caster and play the game to where you need to RELY on spellcasting so that you will be forced to improvise and learn how to cast spells very well. There are also a lot of other tools at your disposal besides strong warriors with big swords, so also look at the items (wands, scrolls, potions, etc.) that are available to spellcasters. Chances are, your character will turn out to be more powerful than you could have imagined.
