[Scaevolus]

The rules you create are in the form of venn diagrams of up to 4 overlapping regions (see shagie's top-level comment), where you can specify rules for each region to match and the sizes of the different overlaps, and then the output is a decision-- marking specific regions as clear/bomb, creating new regions (!), or even changing region visibility-- marking them as hidden or removing them from further rule evaluations.

Those rules can include terms like "5", "x", "x+2", and "x+y+3", so you can express most variants in a signle rule.

The closest to "undecidable" under this rule system is that you can hide a region and prevent further deductions from it, making some board states impossible to resolve. Other than that, all the rules are decidable.