I haven't heard about 135i being confirmed.
My sources never mentioned 135i at all. Yet I4 turbo was mentioned.
I doubt BMW would put a 192kg N54 engine in a poor 1er.
While of course possible new M engine (S55 ??? ) would be much lighter - featuring all the lightweight tech. So if S85 weights 240kg, S65 202kg, then all-magnesium I6 S55 should weight less then existing alu-magnesium N52 I6 - so near, or even less than 162kg.
Also: 192kg engine (despite 306HP / 400Nm) would kill all BMW ambitions of making 1er a true 2002 / E30 successor: small, agile, nimble.
I also doubt 135i & M1 would be able to co-exist.
With N54 1er would be too nose-heavy. I really can not imagine 135i.
BMW never (!!!) have same top non-M engines in different series.
7er has V12, 5er & 6er have V8, 3er has I6 bi-turbo. And I highly doubt they will stick N54 in 1er as well. That will dilute 3er completely. N53 HPI I6 is what will define top non-M 1er, IMO.
Also: N53 HPI I6 engine have some more potential - mind that BMW was able to squeeze 272HP from N52 engine - without HPI. So N53 engine is capable more than currently set 272HP.
So I guess 130i (N53 HPI I6) coupe could get 286HP, while M1 could be set at approx. 350HP. And with no 135i in portfolio.
And I can easily imagine limited edition 120si - same engine as 320si. Since 320si edition is already sold out.