Nascent Open Source Instruction Set Architectures such as OpenPOWER or RISC-V, allow software/hardware co-designers to fully utilize the underlying hardware, modify it or extend it based on their needs. In this paper, we introduce the vision of the MareNostrum Experimental Exascale Platform (MEEP), an Open Source platform enabling software and hardware stack experimentation targeting the High-Performance Computing (HPC) ecosystem. MEEP is built with state-of-the-art FPGAs that support PCIe and High Bandwidth Memory (HBM), making it ideal to emulate chiplet-based HPC accelerators such as ACME, at the chip, package, and/or system level. MEEP provides an FPGA Shell containing standardized interfaces (I/O and memory), enabling an emulated acce...