Seeing this has become a 'what clubs have merged' topic, I'll post which ones from the Hills that I know of:
* Eastern Rangers merged with Uraidla in the late 90's to become Uraidla Districts (It wasn't much of a 'merger' though, I think Rangers would have folded). Uraidla added 'Districts' to their name and became the 'Demons' instead of 'Redlegs' - Rangers went from being 'Saints' to 'Demons'! Uraidla Districts don't play any football at Ashton, the Rangers former home ground.
* The Mt. Torrens 'Bombers' and Pleasant Valley 'Roos' merged at about the same time to become the Torrens Valley 'Mountain Lions'. Their junior sides had merged a few years previously and had been playing as The 'Bomberoos'. This was a genuine merger as they created a 'hybrid' guernsey and share playing venues. Not sure how the committee was formed but the club is obviously still going strong.
* Mt. Torrens/Cambrai had a combined team for a period between the 70's & 90's. I'm not sure on the details, perhaps someone involved could elaborate. I'm not sure on the later Sedan 'Roos' and Cambrai 'Magpies' merger either.
* Nairne/Bremer are a combined outfit from the 50's?? who did a stint in the RMFL and came back to the Hills via what used to be Div. 1.
Milang have come and gone from the Hills on more than one occasion, Lenswood Ranges folded in the 80's?, Woodside Army had a team at one stage (not sure of the dates), and in recent years Kangarilla and Meadows have rejoined the Hills League but I don't know much about their history's except that Meadows were once the 'Peckers'. Correct me if I'm wrong but I think Birdwood, Gumeracha, Kersbrook, Echunga and Macclesfield have remained largely unchanged throughout their history's.
From Central Division:
Mt. Lofty are the product of the Stirling - Heathfield/Aldgate United merger (insert date).
Not sure if Onkaparinga Valley are the result of a merger.
Over the years various junior outfits have been created from a blend of two clubs (usually large clubs with a surplus of kids topping up a smaller club's team) with such names as 'Black Thunders', 'Maccybank' and 'Ironfield' but there are too many to name.