tl;dr
Smaller citadels with limited lifespan
Future update will remove dirt cheap citadels from EVE
While I like idea of removing citadel spam from game this also means that small POS role is not covered by citadels
So back to idea:
Introduce smaller size of citadels with worse defense and abilities (and so on)
Price should be about 150,000,000 - 500,000,000 for various purpose types
and most important
they SELF DESTRUCT in 30 (or less) days after anchor