The piggy back starter idea interests me, but I'm not sure how practical it is.
Let's say you have 4 guys who fit the bill perfectly: Houck, Whitlock, Richards and Pivetta. Do they just pitch 3-4 innings every 5 days? Do you use them in short relief on their "throwing days?"
In theory, it might work best for them to pitch 3-4 innings every 4 days, but that would mess up the 5 day starter routines.
Can a pitcher thrive under this routine:
3 IP
rest
1 IP
rest
3 IP...
Over 160 games, the pitcher would have 40 three inning games and 40 one inning games for 160 IP.
Let's say you have 4 guys that follow this pattern: it would line up like this with 4 IP everyday from 2 players out of the 4.
20 games cycle with 3 five day starters
3-1-0-0 +5 (9)
0-0-3-1 +5 (9)
1-3-0 0 (4)
0-0-1-3 +5 (9)
3-1-0-0 (4)
0-0-3-1 +5 (9)
1-3-0 0 +5 (9)
0-0-1-3 (4)
3-1-0-0 +5 (9)
0-0-3-1 (4)
1-3-0 0 +5 (9)
0-0-1-3 +5 (9)
3-1-0-0 (4)
0-0-3-1 +5 (9)
1-3-0 0 (4)
0-0-1-3 +5 (9)
3-1-0-0 +5 (9)
0-0-3-1 (4)
1-3-0 0 +5 (9)
0-0-1-3 (4)
Of course these numbers would not come out perfectly, as pitchers might get yanked early when doing poorly or left in longer when doing well, but if it works out something like this, we are talking about 3 starters (5 IP x 33 starts= 165) and 4 pitchers with 160 IP (see above).
That leaves 5 IP needed from the pen in 3 of every 5 games (days). If we have 6 pen arms, that's 15 IP/ 6 pitchers every 5 days.