What other primitives are needed to build a rock solid DeFi lending protocol?

Here's mine:

Secure cross-margin, fixed rates that'll attract enough supply side demand, "margin call" grace periods for added liquidation protection, cross chain loan execution for max liquidity efficiency, embedded protocol insurance against hacks, onchain lending order books (e.g. bitfinex) for trading, user "opt-in" rehypothecation, support for RWAs and design all of this with careful tax consequence planning.

What else?