I’d happily include PPC and XPM for technological reasons in that index.
PPC can be regarded as a kind of ‘insurance’ against BTC failure. PPC as old PoS coin with a track record might be seen as safe harbour in case BTC fails. I don’t know about its potential independent from BTC failure. PPC trade volume is quite low. It should have only a minor role in the index.
XPM is quite ASIC resistant. Are there even FPGAs? I’m only aware of GPU miners. The inflation model is smoother than Bitcoin’s and the number of tx per time can exceed Bitcoin’s.
But I fear there’s not really enough trade volume in XPM pairs to consider it for the index.
To float an idea possible members of such an index could be (part 1):
BTC
ETH
XMR
LTC
FCT
XCP
DASH
NXT
MAID
BTS
Optional members with low percentage (due to design or trade volume (part 2); or candidates for a second, ‘increased risk’ index):
WAVES
EMC
SJCX
SYS
NSR
XPM
The choice is somewhat randomly taken from listings on coinmarketcap, although I tried to include coins with a reasonable trade volume and features (vastly/slightly) different from other index members.
Part 2 obviously deals with a set of coins that should have only little percentage (due to little trade volume), but could play a role (due to features) in an index - maybe a separate index.