1. Versions matter - she'd often make a draft and want to tell me "ok tell me if you like this better than the previous version". Id just compare the cells above to the cells below.
2. We organized people based on common interests - these people are very into music, these are very into home renovations, these people work in tech...etc. Tagging people with "attributes" and letting the system try optimize for as much overlap as possible would have been amazing.
3. Warn me of duplicates - sometimes in the copy paste madeness, we'd double up someone.
4. Import - We almost missed someone due to me skipping a line in the initial transription of their name into the spreadsheet.
5. At the end of the day guests need to find their seat. I made a QR code that linked to a website where people could type their name to find their table (https://andrewmcgrath.info/wedding) we didn't want to use a big board, it was 1 more thing to make (the website was a 2 hr project i did the day before) and the QR codes were just print outs we put in dollar store photo frames at the entrance, the bar and at the end of the day...we got great feedback over this. With the guest lists, you could easily automate making someone this website.
Good luck!