One of the reasons for the lack of variety in weapons is the very large scale 'grid' used in combat. It's only about 20 squares across. If we could have a much finer grid in the combat screen we could have as greater variety of weapon ranges. Hopefully the combat system in SE V will be 'coordinate' based and allow finer variations.
Another problem is that all weapons lose accuracy at the same rate. This doesn't make sense from a realism standpoint. It should be much harder to make a hit with a DUC at long range than a particle beam like the APB. It's highly unlikely that a DU projectile could be accelerated to the speed of a particle beam - or at least it would be vastly more difficult and so reserved for a higher tech level. So there would be much more time from launch for a target at long range to move or use some counter measure to avoid the hit. Some weapons might be 'instantaneous' and not lose accuracy with range at all, for that matter. But neither of these options is possible in SE IV. If we could set the accuracy loss per unit of range individually for each weapon we could have some very interesting variations in weapon types. Another thing to hope for in SE V.
There are more damage types than are actually used in the default tech set, so people can mod some good variations if they want to. I suggested that the Acid Globule be given the 'Quarter Damage to Shields' damage type, for example, because acid is not likely to be very useful against shields. But you could then crank the damage up to much MORE than other weapons of the same class since the acid would be far more effective against physical objects than many particle beam weapons. Some of the original damage types are just not used very much, like 'skips armor and shields'. More than a few mods have given this ability to the Graviton Hellbore to make it worthwhile to research.
I've been asking for more damage types for a long time. Persistent damage that carries over from one turn to another (like the 'plasma web' from MOO 2) -- which I'd prefer to give to the Acid Globule. Levels of armor hardness and armor skipping to make a more interesting 'arms race' similar to cloaking and sensors. Even percentages of shield and armor penetration instead of 'all or nothing' like we have now. I suppose there are limits to what can be done without rewriting the SE IV combat system.