In order to justify how much work there is to be done to support Zcash ecosystem security, here’s a list of projects I’d consider to be in scope and would probably be worth prioritizing in the first year.
Note that I haven’t started digging into any of the code or spoken with the developers in order to work out a relative prioritization yet. Prioritization would be based on a number of factors I mentioned above such as project activity, apparent quality of the code, amount of users, overall risk, etc.
Unless otherwise stated, I’d say all of the following would benefit from the full 10-day audit.
Edit: I’ve later added some things suggested in replies.
Community Wallets
Wallets are on the front lines, and since they are responsible for storing users’ funds, they are what will expose the most users to the most risk, so they should be our highest security priority.
- ZecWallet
- ZecWallet-Lite
- I’d especially like to review the advancements in syncing and other major differences between this wallet and ECC’s wallet code, as well as the future work to add Orchard support.
- NightHawk
- Unstoppable
- Edge
- Zephyr
- I’m really excited about Zcash in the browser! Browser security models are “fun”!
- Ywallet
Crypto-Heavy or Privacy-Related Stuff
I love crypto so these will be really fun to audit. 
- Shielded Trezor Support
- Arti, to the extent that it’s used in Zcash wallets.
- Especially discovering any privacy leaks that are unique to mobile environments.
- ZSA design and future implementation
Things Built on Zcash
My vision for Zcash is to become a platform for developers to build awesome things on. That way, we can reach into totally unexpected use cases beyond payments and wallets. Eventually one such project will turn out to be a hit, and we’ll want to make sure it’s secure.
- Zbay
- ZECpages (5-day audit is probably sufficient)
- free2z (5-day audit is probably sufficient)
- …and lots more to come, I hope!
Supporting Infrastructure
- lightwalletd hosting and maintenance
- Here it would be useful pen-test the actual running infrastructure and suggest design improvements like more-private logging, using sentry nodes to protect the main zcashd node, and ephemeral instances to reduce the blast radius of attacks intending to compromise users’ privacy.
I’m probably missing a lot! @ me if you’d like to see your project added to this list. I also had the opportunity to skim through the grant application list and there are a bunch of cool ones I hope will be funded in the future, like shielded multisig once FROST is available.