I took a quick look at my files regarding this topic, looking at current peers (Dukes, Marquesses and Earls/Countesses only) and their relationship to their heirs, both apparent and presumptive.
I eliminated all titles were there was an heir apparent, and just looked at the heirs presumptive. Within that category, I wanted to focus on titles where there was a "cousinship" or "kinship" relationship, so that eliminated the hp's who were siblings, half-siblings, uncles, nephews, and grandchildren.
Here is the list I came up with (number of the peer is in parentheses) (I was a bit pressed for time, so the list is not exhaustive. Also, my apologies for any errors):
DUKE
Westminster (7th)-- Heir to the Marquessate of Westminster is a 3x great-grandson of the 1st Marquess )
MARQUESS (stated in a previous post)
Abergavenny (6th)-- Heir to the Earldom of Abergavenny is a 2x gt-grandson of the 4th Earl.
EARLS
Arran (9th)-- Heir is a 4x gt-grandson of the 2nd Earl.
Buckinghamshire (10th)-- Heir is a 3x gt-grandson of the 3rd Earl.
Cavan (13th)-- Heir is a 7x gt-grandson of the 1st Earl.
Chichester (9th)-- Heir is a gt-grandson of the 5th Earl.
Craven (9th)-- Heir is a grandson of the 3rd Earl.
Enniskillen (7th)-- Heir is a gt-grandson of the 4th Earl.
Erne (7th)-- Heir is a gt-grandson of the 4th Earl.
Essex (11th)-- Heir is a 5x gt-grandson of the 4th Earl.
Fortescue (8th)-- Heir is a grandson of the 6th Earl.
Gosford (7th)-- Heir is a grandson of the 5th Earl.
Home (16th)-- Heir is a gt-grandson of the 13th Earl.
Morley (7th)-- Heir is a gt-grandson of the 3rd Earl.
Onslow (8th)-- Heir is a gt-grandson of the 2nd Earl.
Shannon (10th)-- Heir is a gt-grandson of the 5th Earl.
Brooke