A TF card (microSD) + card holder is fine to add, but it changes the design in a very specific way: it gives you removable flash storage, not system RAM.
What it’s good for
Boot media / OS storage for embedded platforms (if your CPU/module supports booting from SD)
It does not replace DDR5. A TF card is orders of magnitude slower and not memory-mapped like DRAM in a typical PC-style system.
Key design implications (schematic + PCB)
Interface choice: the host must support either SDIO (4-bit) or SPI mode.
Voltage levels: most microSD runs I/O at 3.3 V (some hosts use 1.8 V signaling for high speed). You must match what your host supports.
ESD protection: add ESD on the card-facing lines (CLK/CMD/DATx) and the connector shell.
Power switching: it’s common to add a load switch (or at least a ferrite + decoupling) so you can power-cycle the card.
Layout: keep CLK/CMD/DAT short, matched where needed, and avoid stubs; place the connector at the edge with a solid ground reference.
If you tell me what will “host” the TF card in your project (a specific CPU/module, or “just for a microcontroller later”), I can tell you whether to wire it as SPI (simpler) or SDIO (faster), and what extra support circuitry you’ll need.