Can't really do 5-6 new series for each SRW, not enough series and too much money spent on making sprites, lol.
It should try not to reuse series more than twice if it's been in an SRW in the last 3-4 titles if anything.
Well, let's look at debuts (if something hasn't been in a SRW in like 10 years I'll still count it):
Z1:
New fresh series: SEED Destiny, E7, Gainer, Gravion, Big O 2nd Season, Zeta Movies
Classic series: God Sigma, Baldios, Orguss, maybe Gundam X & Turn A because they're kind of old nostalgic.
So even if you don't really count Zeta movies or Gundam X/Turn A, it's 4 newer series, 3 classic series, then a bunch of mainstays.
K:
New fresh series: Star Gazer, GunxSword, Zoids Genesis, Fafner, New Jeeg, New Gaiking
6 Newer series, 0 classic debuts, a some mainstays.
L:
New fresh series: Eva Rebuild, Linebarrels, Mac F, Dancouga Nova
Classic series: Iczer one
4 newer series, 1 classic; bunch of mainstays.
Z2.1:
New fresh series: Gundam 00, Code Geass, Gurren Lagann, Shin Mazinger, Dai-Guard, E7 movie, Mac F movies
Classic fresh series: Votoms, Godmars
That's 7 newer series, 2 classics
Z2.2 doesn't really count
UX:
New fresh series: Heroman, Demonsbane, Wings of Rean, Fafner Sequel, 00 Movie, Mazinkaiser SKL, Linebarrels Manga
Classic series: SD Gundam 3 Kingdoms
7 newer series, 1 classic series
Z3.1:
New fresh series: FMP Novels, Aquarion Evol, Unicorn
Classic series: 0
3 newer series, 0 classic series. Mainly OG plot.
Z3.2 - doesn't really count
BX:
New fresh series: Gundam Age
Classic series: Giant Gorg, Panzer World Galiant, SD Gundam Gaiden, Raijin Oh
1 new series, 4 older series.
Looking at it, there's not really much of a pattern tbh. There's usually about 6-8 debut series with some ratio of new stuff/old stuff and then mainstays, sometimes with new sprites if they haven't been in a SRW in a while.
I think I like BX because it's got a good balance of classic robot shows, and the last SRW to really do that was Z1. Then again I liked K and Z2 which were jam packed with fresh series, so both work.
Maybe 4 fresh series, 3 classic series per entry would be good.
And there are TONS of shows left to debut in SRW because they've been coming out slowly lately and only adding a few shows at a time. The 2008-2013 era added a bunch of new mecha shows post Z2, but UX/Z3 barely used any of the new ones, and cell phone debuts do not count.