## 5 Open Challenges and Future Directions  
By considering what has been discussed and summarized in the previous sections, this section identifies some open challenges and proposes some future directions. The main challenges that still need to be tackled are essentially related to finding the right compromise between the costs of storing data on-chain and the level of security that can be achieved with off-chain storage. In particular, we can summarize the main challenges with the following points:  
Cost--The high cost of storing data on-chain is a significant challenge for the broader adoption of rollup solutions, making many rollup solutions less economically viable than their L1 counterparts. For instance, storing data on calldata can cost 16 units of Gas for non-zero bytes and 4 units for zero bytes. Blob aims to reduce on-chain storage costs. However, in some cases, such as during periods of high demand, blobs can be more expensive than traditional calldata, preventing rollups from implementing blob-based solutions. Moreover, another significant challenge in the blob is that rollups have to pay the full blob fee regardless of the data size, which can be expensive for smaller rollups. In this case, rollups need to wait until they have enough data to fill a blob slot, potentially delaying transaction finality or submitting smaller data segments, increasing user costs. Blob still needs to improve in terms of efficient implementation and scalability. Security and Trust--The security and trustworthiness of data availability solutions are crucial. The decentralized solutions aim to reduce dependency on trusted third parties; however, these solutions still face new security challenges. For instance, DAC relies on a few trusted nodes, which poses a risk in case of malicious or compromised nodes. Moreover, data withholding attacks are significant challenges in the data availability layer, where malicious actors can withhold data to disrupt the rollup. Furthermore, maintaining the integrity and verifiability of off-chain data is still challenging, as it requires robust mechanisms to ensure that data are always available and valid. Governance and Regulation--Another challenge is the constantly evolving governance and regulations of blockchain systems. Blockchain technologies and their applications have to handle increasingly complex regulatory and governance rules, which can ensure compliance while maintaining decentralization and user trust in the system. Therefore, rollup solutions need to adapt to regulatory challenges and incorporate robust governance mechanisms to address these dynamics. Interoperability--The interoperability between various data availability solutions and blockchain networks is essential for developing a robust ecosystem. However, several technical and standardization challenges exist in achieving interoperability in the blockchain ecosystem. Furthermore, different blockchains and rollup solutions often employ different protocols and data formats, which can further complicate the efforts needed to ensure the interaction and integration of multiple rollups. Therefore, standardized frameworks and protocols for interoperability are still an open challenge. Given such main challenges, several future research directions need to be investigated to improve the quality of L2 solutions from an economic, performance, and security point of view. Robust and Scalable Solutions--Future research can focus on developing more robust and scalable data availability solutions. Off-chain data availability techniques, such as more decentralized and robust DACs and DALs, can also be crucial. Moreover, new data repetition and identification methods can improve storage efficiency and ensure that unique transaction data are posted to the blockchain. In addition, novel techniques, such as enhanced data compression, erasure coding, and advanced cryptographic methods, could significantly reduce Gas costs and improve transaction throughput.