Four 12s?
I would like to try four 12 inch speakers, two Bass & two mids arranged cruciform. With the cabinet being around 38 inches wide, the two Bass speakers could be pulled out to the sides making room for the two mids to stack up in the center.  Four 12s have area midway between two 15s & two 18s. But with lighter moving mass in each speaker, presumably the motor can exert better control of the cone for a faster, cleaner sound.  
With the big surface area, I guess the Imperial would still have the "hit", the crack! that comes from the twin 15s.  

I was thinking of using Weber guitar amp speakers which are available with 32 Ohm impedance.  The four of them could all be paralleled for 8 Ohms.  

For a tweeter with four 12s, I figured use the same wood horn as with the two 15s.  Although a lateral array of 2 or 3 bullet tweeters would be less work.  The center one would point straight forward, while the outer pair would be splayed outward a few degrees for dispersion since bullet tweeters do not otherwise have much dispersion.

These bullets could mount between the two midrange woofers if these were pulled to the top & bottom edges of the baffle.

Perhaps this tweeter array ought to be on a board which can be rotated up or down so that if they are too intense they can be pointed partially toward the ceiling or floor to get them out of the face of the listener.