Both the Delta IV and Atlas V families of boosters use an interesting concept - common core vehicles. For the Delta IV, the core is the same regardless of whether you're talking about the light, medium, or heavy lift versions. This simplifies construction a great deal at the expense of added structural weight, especially for the lighter versions. The concept is to have several versions incorporating different sets of upper stages and strap-ons to create a family of boosters, as discussed near the bottom of this page

What you're proposing is to create non-common core vehicles in order to create a really heavy lift version. The extra strap-ons you're discussing would mean the center vehicle would have to be stronger to handle the extra thrust as well as the additional payload mass. You'd also have to build a new launch pad to handle the new strap ons.

There may be a simplier way to achieve higher lift to LEO - use solid-fuel strap ons. The common core vehicles are already designed to handle up to 4 GEM-60 solid rocket strap ons. It might be possible to put from 1-3 of those strap ons on each of the outer cores, perhaps with 2 more on the center one. Structural loads would be the limiting factor as well as launch pad configuration, but it might work.

A couple months ago, I saw a set of proposed growth versions for the Delta IV Heavy that incorporated ideas like the ones I've mentioned. Unfortunately, I can't find a link to the site.

Of course, if you really want heavy lift, the perhaps we should consider restarting the Energia production line. The hardest parts (the engines) are still in production. You'd be talking about building new central tanks and perhaps a new launch pad closer to the equator.