I think the whole souvenir thing could use a big revamp... My interpetation is that the purpose of souvenirs are to show off that you've achieved something (visited a certain state/country, cached on a certain date etc.). Why not look at other simillar systems with the same purpose and borrow some ideas.
For example World of warcraft (or any of Blizzards new games), the Sony playstation trophies or even the BadgeGen-macro used by many Geocaching users.
For example... looking at World of warcraft and translating it to geocaching I would suggest dividing souvenirs into two groups (with sub-catrgories): souvenirs that allways can be obtained (ie. "Find 100 traditional caches" or "Found a cache in Florida"), and souvenirs that are no longer obtainable (ie. "Find a geocache on August 1st 2013").
Since alot of geocachers (as least my experiense so far, and myself included) are somewhat nerdy (I mean this in the best possible way), numbers and statistics are appealing, so I really think a more extensive achievement system would be really well received.
With that said im currently working on my 31 Days of August streak... so far so good.
Just my 2 cents.
