Expansion Possibilities For The Proposed HLV

Modern rockets use SRB's as "strap-ons". The Heavy Lift Vehicle has plenty of room for additional SRB's.

Adding another pair, for example, would add another 6.6 million lbs. to the total sea level thrust. 8.69 + 6.6 million = 15.29 million lbs. There could be room for additional strap-ons beyond that.

A further consideration is that strap-ons are not necessarily limited to pairs. There could be a third strap-on, or a fifth, and so on. The strap-ons would probably need to be arranged equicircumferentially (equally distanced from each other along the spacecraft's fuel tank circumference).

Where more than two SRB's (total) are used, the structure of the spacecraft would probably need to be reinforced, unless the design was initially sufficiently overbuilt.