Are there any options for new rear suspension?
Yes. The shocks from the VTX 1300, 1800, and Valkyrie (non Interstate) are all bolt-on options to the Super. If you use any of these shocks as they are, they will lower the back of your bike a couple inches. In order to maintain stock height as much as possible, you will need to swap out the shock tops with those from the Super (You will need to make or borrow a spring compression tool to do this or have a dealer do it for you). Also, if you decide to use your new shocks as they are, you will need to slightly bend-out the leading edge of your grab rails in order to get the proper clearance.

So how do I change my shocks?
It's pretty simple, and many have even done it without jacking-up the bike. Most of the project is straight-forward except for the lower nut on the left side. Loosening this nut will occasionally back out the bottom mount (stud) to which it is attached. To avoid this, hit the end of that acorn nut with a hammer a couple/few times (Protect the nut so you don't damage it). Then, put your socket on the nut, gently tighten, then quickly jerk it loose. You may have to do this several times before it will come off without also removing the stud/mount from the drive housing.

Valkyrie shocks with Super tops/ as is = Near stock height/Noticeably Lower.

VTX 1800 shocks with Super tops/ as is = Noticeably lower/Very Low.

VTX 1300 shocks with Super tops/ as is = Low/Slammed.

Valkyrie or VTX 1800 shocks with Super tops would be the best choices for 2-up riding. VTX 1300 shocks even with Super tops would not be the best choice for 2-up.

Part Numbers and dimensions.                                                                            
VTX 1300 = MEA-671,  12 1/8" center to center (all models) and 642 lbs.
VTX 1800 C = MCH-003, 12 1/2" center to center and 705 lbs. (dry).
VTX 1800 R = ???-671, 12 1/2" center to center and 718 lbs. (dry I believe).
Valk standard = MZO-A41, and MZO-003, 13" center to center and 680 lbs. (dry).
Valk Interstate = MBY-671, 13" center to center and I believe 710 lbs. (dry).
Stock Super = MNO-003, 13 3/4" center to center and 510 lbs. (dry).

NOTE: Some of these part numbers may have been superseded, as they are a few years old. My experience has been that you will usually be directed to the new part number if using any decent search function.

Just remember, when putting the Super Shock tops on any of the above shocks, you'll add 3/4" to any of them, they all have the same shock perches. The Valks with the Super tops will keep you closest to stock height.