Many businesses need software that's almost right from existing solutions, but require custom tweaks that are too expensive to build from scratch.
Offer industry-specific SaaS foundations (e.g. for ecommerce, scheduling, CRMs) with well-designed extension points and clear customization guides.
Charge monthly for the base platform plus professional services for customizations. Target small businesses with specific needs.
Start with one vertical where you have expertise. Build the most common extensions as optional modules.
Risk: Maintaining balance between flexibility and stability as customizations multiply.