There are some popular advanced mathematical textbooks that needs serious modernizing.
Such as formatting dull looking texts into colorful blocks, bridging explanations & terse proofs that shouldn't take 100 minutes to engulf, conceptualizations for better understanding, and enthusiastic informal tones for the sake of reading motivation.
Here are some planned math textbooks that needs serious modernization:
Calculus - Michael Spivak
Mathematical Analysis - Rudin
Real and Complex Analysis - Rudin
Abstract Algebra - Dummit & Foote
Algebra - Michael Artin
Topology - James Munkres
Algebraic Topology - Allen Hatcher
Princeton Lectures in Analysis Series - Elias & Rami
Visual Complex Analysis - Needleham