Many participles in Latin are "substantivated": that is, they effectively become nouns - using the neutral gender either by plural of singular like secretum or apertum. Do we have examples of gerundives - which are also types of participles - become like nouns?
Can the Ciceronian quote: "cavebo quae sunt cavenda" be shortened to cavebo cavenda? Or else, it is impossible grammatically?
As noted by Mitomino in later times it appears to be possible. I think I've found another non-classical example: et bene puerilem ostendit animum, qui per mobilitatem et inconstantiam frequenter optat timenda (Serv.A.4.157.5).