In a d20 based system, there's a sweet spot for modifiers around the +10 mark - that's the point where the final result on a check will be equally dependent on 'luck' and 'skill'. (Mathematically speaking, d20+10 vs DC 20 is the same as d20+50 vs DC 60... but the former feels better.) Ideally, you probably want the total modifiers used across the full campaign to be in the range -5 to +25 (at the extremes), with most of the play taking place in the +5 to +15 range.
Additionally, there's an issue with specialists and non-specialist characters, and the gap between them. If the game provides lots of options for customising characters (as in both 3e and 4e, and most likely 5e), this gap will inevitably increase with level, probably quite sharply. In 3e, it got to the point that people with a 'good' save could succeed on anything but a 1, against attacks that people with 'poor' saves needed a 20, which was suboptimal, to say the least.
That suggests to me that the correct starting point for the specialist (the Fighter with his chosen weapon) is probably about +5 at 1st level, for the non-specialist (the Cleric with a melee attack) is about +0, and the anti-specialist (the weakling Mage with non-proficient weapon) is about -5.
Then, give the characters a modest automatic increase with level. In a 20-level system, +1 per 2 levels is probably about right, while in a 30 level system +1 per 3 levels is probably better.
And then give characters plenty of options to advance those areas they care about.
That would mean that at the outset, the characters who are 'good' at something are already pretty decent, while characters who are 'bad' at something are truly lousy (as bad as the system really supports). Then, after just a few levels, characters advance into the 'sweet spot' for modifiers in the areas they specialise in.
And, ideally, as they get to the top end of the level scale, their permanent modifier in their areas of specialisation is about +20, which remains reasonably controlled, while still allowing for a bit of breathing room to add on some situational bonuses/magical buffs/whatever to take them up to the +25 that is the top end of what the system easily supports.