I don't think it would be too overpowered. To be honest, on a lot of games you're going to have to settle on other continents to get full use out of the Candi anyway, and having them all on the same continent would make it far easier for Pangaea. As a complete change, getting gold from trade routes that pass through your territory would be nice.
Well, I don't know if you are aware of the consequences of your proposal. If the first 3 cities would give, without restrictions, the 3+3 luxuries , then I would definitly apply the following warmonger strategy:
*Build as soon as possible 3 cities, the luxuries would compensate the unhappiness part
*trade away the copies for 15 gpt and maybe even 1 more luxury for a total of 20 gpt
*build many composite bowmen and some melee units (funding the unit upkeep with the trade income)
* attack (possibly together with some other civs for less warmonger penalty) and capture a capital located on a nice spot or which has build early wonders you want to have
* you eliminate/cripple a direct civ early on and can continue when the kriss swordmen come around for the 2nd wave
But back to reality, with the candi change you get 2 benefits from the UB. First is you can get 2 faith and +2 faith per religion with a follower in
every city with candi. Second you can have the %25 modifier for great people generation in
every city. I can imagine you can easily get an average of 2 religions per city for a total of 6 faith producing candi per turn. Compare the candi, producing 6 faith for 120 hammers which also provides great person generation modifier and is buildable in every city, with Stonehenge which just give 5 faith per turn for 185 hammers only in 1 city. Indonesia should definitly be played more religionbased now.
For your suggestion of changing the UA of Indonesia to something related to trade routes passing through their borders. Well it definitely is a good idea. But with the expansion already containing 3 civs (Venice, Portugal and Morocco) with a trade route related UA and Arabia's UA getting an overhaul trade route related, the chances were slim that they would introduce another civ with similar UA I think.