You can't make Dash into a side-chain, it's immune to that. The economic model won't function properly on a side chain and whole thing will be insecure. Who's paying the masternodes an incentive, where is that money coming from?

Dash can be made into a sidechain. The PoW function is simply replaced with a two-way-peg to BTC.
The advantage of that is BTC's well-tested sha256 network hashrate is orders of magnitude higher and thus more secure than Dash's problematic homebrew X11.
Sidechains may set whatever local rules they want, including PoS schemes for Masternode voting.
Therefore a sidechain is the ideal medium for a fair Dash relaunch, to take care of the X11 weakest link, Insta-mine, and Foundation centralization problems once and for all.