I've often considered one of the sources in the dramatic variation in difficulty to be the compounding nature of super-effectiveness and STAB, wherein there is an all-too-easy way to accumulate attack bonuses and, all other things being equal, battle becomes punishingly hard without a STAB and/or super-effective move. In this line of thought, I've often considered using single-type Pokemon to be overly self-punitive; a shame since so many of such mons are appealing.

To resolve this latter issue (the former would need far more attention), what if STAB was much more contingent on the type of Pokemon using it. That is, generally, types that intuitively would favour one or more types not of its own nature would get a moderate S(imilar)TAB; Ground types would have a 25% damage bonus when using Rock type moves (contrast to the 50% bonus Rock types have), for instance. Might this make single-type mons/mons of less useful type combos overall more viable? Or would it just result in an unwieldly power creep for dual types on its own?

Pic somewhat related.