I've been giving it some thought. Technology is the obvious answer. They are no perfect safeguards, and there will need to be manual processes for the foreseeable future to accommodate those who are technically challenged.
First, we should try to eliminate mail in votes and absentee votes. These rely on the mail, and there are different rules about when they must be postmarked, received, etc. I think the answer is online voting. The potential issue with that is identity theft, and fraud. I think the solution is having a dedicated site and secure site, and issuing a security token, and validate a biometric factor like fingerprint or eye scan.
If people want to still mail in a paper ballot, that would be fine, but it would have to be RECEIVED by election day, by mail or by drop box. That would eliminate the date and postmark scrutiny. Having an electronic portal solves the issue of Military mail in ballots, and just sketchy USPS wait times in general.
Internet security would be the major factor overall. Local governments aren't the best at that. Heck, even the Defense Department has been hacked. There are ways to deal with that though.