what calibers do you have now? is there any specific thing you want this next gun to do? ie - hunting, accurate, long range, etc...
for me i used 1 lower and SBR'd it. then put a 5.56 upper, 300BLK upper and 22LR upper for it. kind of like 3 SBR's for the price of one.
even if you don't SBR it, you can still do 5.56, 300BLK, 22LR, 9mm, maybe even a 6.5 or 50Beowulf.