I find the root reason of lacking of BTA 
all the BTA is created by loan,  it mean all BTAs are debt by collateral,  there no way to create base currency.  
so alway there is lack bta ?
so I suggest that can destroy BTS to create BTA as base currency.
My proposal:
notice all the price is (BTS/BTA)
1.there are two condition LACK or OVERFLOW  respectively describe BTA is lock and overflow. 
   1.1  if feed price is higher than sell price of BTS ,it mean BTA is on LACK
   1.2  if feed price is lower than sell price of BTS ,it mean BTA is on OVERFLOW 
2.if BTA is on  LACK, BTA can been created by destroying BTS  at 0.995* feed price directly , and the max amount BTS that can been created = all sells that price is lower than feed price .
3.there is a pool of destroyed BTS, all destroyed BTS is go to this poll 
4. if BTA is on  OVERFLOW , the BTA holder can directly destroy BTA and get BTS from  pool of destroyed BTS  at the price of 1.005 *feed price.