NSU suffer from two different issues
- lack of variety of capabilities (which, to be honest, is an issue with many fantasy RPGs because, apparently, it's much easier to design a hundred new spells than a hundred different new ways of useing a sword, who would've thunk ?
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- asymptotic effectiveness, which is a bit more RM-specific : power increases linearly with level on spell lists, while skill bonus is not linear per level.
The second point actually has potential for NSUs : it means they can allocate more DPs elsewhere, while the efficiency of a spellcaster is more or less tied to their continued investment in spell list acquisition. However, fighters get the short end of the stick because their speciatly is combat and skilling up different weapons do not really makes a fighter more versatile, and their development for noncombat skills will always suffer when compared with other classes.
I'm not a fan of adding even more skills of very limited scope - if anything, RM has too many skills already IMHO. Allowing different uses of skills according to the number of ranks developed makes more sense, especially for combat skills (capability to perform a sweep attack with a large weapon, to trip an opponent with a pole arm, to perform combat manoeuvers to gain positional advantage or force the enemy to fall back, and so on). Of course, having said that, you will likely look at noncombat skills and wonder if they can also benefit from increased scope.
The first point is something you cannot really work around, because it is encoded in the way magic is defined in the game (some games have magic systems with a much narrower scope, which more or less cancels this issue, but RM - among others - suffers from it). The only way for Fighters to compete is... to acquire spells. Not combat spells (except possibly self buffs), because Fighters in combat have a well-defined role which is not that of a caster, but spells that provide them with an edge in non-combat situations. One way to do that is to reduce the cost of spell list acquisition for NSUs, especially for the lower level spells. Even a handful of Open Lists can make a world of difference.
The downside of this solution is that you basically acknowledge the fact that Magic is the be-all, end-all at higher level and just decide to roll with it. But the alternative - reining in magic at higher levels - requires a much more extensive redesign of the game.
Note that if the setting has a built-in limitation for casters (such as any serious late 3rd Age Middle Earth setting), then balance becomes much less of an issue because the casters have so many spells... they basically can't use except in life-or-death situations. In those settings, you will likely see spellcasters spend a great deal of their DPs on mundane skills... which solves the problem in a different way.