Most people design siddurim with some kind of innovation in mind. Their claim to fame is the "hiddush". The role is to fill a niche. The need is immortality - to leave a legacy with their own personal stamp.
I plead guilty to many of these, but to me a Siddur is a classic text like Torah or Mishna and therefor an edition should be more about being faithful to the classics than being innovative.
I am going to forgo the issue of translation. For now design is more about a good scholarly or critical edition.
The best scholarly Siddur I know of is the Seligmann Baer Siddur Avodas Yisrael.
My gratitude to Belz School for introducing me to this modern classic.
Along the lines of this Siddur I would have the following components
1 All sources given (meqoroth). Even paraphrases of Miqra, Talmud and Midrash should be footnoted.
Artscroll does a good job on this
2 All Ta'amei Miqra should be restored. This was prevalent in Yekke prayer books until about a century or 2 ago. It is still popular in siddurom from Edoth Mizrach
3 Physical formatting of piyyutim and other rhyming schemes, acrostics and the like should reflect the poetic structure and verse. Koren is really the winner hands-down in physically formatting the page layout to give a visual representation of the Prayers.
4 Preferably mainstream nusach should appear on the text w/o variations. Variations within the text are often confusing. Shinuy nuisachos should be footnoted on the bottom of the page.
5 Optional: an appendix of how certain nusach has "evolved" comparing earlier sources with contemporary ones (masoroth).