I was doing some poking around on
AASHTO's route numbering archive looking to find any interesting proposals or submissions from Delaware I hadn't heard about and ended up finding just that. Some background to set the stage:
Back in 1978, DelDOT had this nagging concern that routing the mainline interstate through downtown Wilmington was causing too much thru traffic to intermingle with local traffic, causing congestion that could be easily resolved by getting the thru traffic to use I-495 instead. On top of this, they were preparing to tear the Wilmington Viaduct on I-95 apart for a massive rebuild and were afraid of apocalyptic congestion (not unlike the current viaduct rebuild project). Their solution, as Alex has already documented on the main AAR site, was to move I-95 onto the Wilmington bypass route (I-495) and redesignate the downtown route as something else.
Their first idea, submitted to AASHTO on May 31st, 1978, was to number the downtown route I-595. AASHTO approved of the general idea, but told DelDOT to choose either I-495 or I-695 to follow loop / spur numbering rules.
Rather than follow through on this, DelDOT decided to split the downtown route into two designations, I-195 between the Christiana Marsh interchange and US 202, and I-395 between US 202 and Pennsylvania. That apparently wasn't all there was to the proposal apparently, as
DelDOT's full plan was to extend I-195 up US 202 / Concord Pike to the intersection at Powder Mill Road (what was then DE 141). Applications were sent in on September 18, 1978 for I-195, I-395, and what was dubbed Temporary I-195 over Concord Pike, and all three were accepted! The Temporary designation seemed to be in place as DelDOT acknowledged it'd take them some time to bring the southern reaches of Concord Pike up to interstate standards.
Their plan would've looked something like this:

Sometime in 1979 DelDOT decided it didn't like their previous idea, so they retracted the I-195 / I-395 plan, along with any plan for an interstate designation along Concord Pike, and went with the I-895 plan that they eventually followed through on. That was reverted back to the I-95 / I-495 setup as soon as construction in Wilmington wrapped up, as evidently DelDOT's concerns of thru traffic clogging up the downtown route evaporated (or someone in Wilmington complained).
The idea to designate a portion of Concord Pike as an interstate doesn't appear to have come up anywhere since, and this is the first time I've ever seen any mention of the idea. I thought it was a pretty neat find!