depends really... on how you want the party dynamic to turn out.

Dungeons and Dragons sell the party as a party; small cogs trying to work together in getting things done that none could do alone. the wizard is frail but powerful. the fighter can take a hit. the cleric can heal. depending on edition they are well developed classes but that forced development into a niche limits characters without branching out into traits or flaws or feats... whatever it is that you can do to separate your character from every other character of that class.

D20 modern went is a different way with base classes. "[ability score] class" of which there was six varieties and you can kinda guess what they were. Tough heroes (those who had a good constitution) were simply tough; supernaturally so with elemental resist and damage reduction built in. the levels capped out at ten so you had to multi class into an "advanced class". each book had a number of new classes (always at least one for each focused hero and a few for the unfocused) and depending on your choices you can multi class into. nearly all requirements could be gained just from skills and feats so the beginning choices in class and ability score didn't matter so much as getting the class skills or just ranks.

the problem is a bit of sameness where no one really stands out both from each other and most human enemies which helped with the modern feel. But if pressed the game can advance with just one maybe two people as a team and build in weaknesses are simply not a thing; though you will miss the healing a cleric can provide.