@Dioxine, I think you make some really good points.
I've been thinking about how could I recreate the Buck Rogers SSI Gold Box RPG or some other Sci-Fi like RPG with the OpenXcom engine, and a lot of the pieces are already there. In fact, I think more will be there once TFTD support is finished. Shoe's soldier diary mod really enhances the "character" development side at least as far as game progression goes by giving the soliders a "history". One of my favorite mods! Which I think hits on the most "lacking" part of the OpenXcom engine which you already hit upon - character development through customization. Again, I'm not knocking OpenXcom in anyway, because I understand the goals it is designed for in the first place - recreate the XCOM experience and a wonderful job that is being done. So, I know I'm getting outside the design when I talk about this.
So, I wonder if adding more "abilities" like lock picking, bypass security, programming, piloting, fast talk, disguise...a bunch of others with custom made missions that need those skills to succeed would create a more RPG experience. Also more classification of character occupations in a mission as well, like I need to take this engineer on this mission to reprogram this computer at this super secret enemy base, and we need to be stealthy and not alert enemies of our presence or I actually recruit real pilots with piloting skills to man my crafts.