• removing all "big" dependencies : I know it has been done with xtext (which means removing the DSL, and probably move the configuration and the rules in UI, java code or the new rule engine) but there are probably other libraries that could be removed : guava, guice, jersey, ... (these 3 only amounts to one third of the size of the minimal OH2 runtime)
  
• run on a smaller OSGI Framework : I've read that concierge was a potential target, but is that already a reality ? I don't see a lot of activity on the project and I could not find any information on it's current status. Knopflerfish could also be an option, has anybody already tried it ?
  
• run on a smaller JVM : either javaSE Embedded 8 (any luck on that ? ) or any of the other embedded jvm ?
  

• removing all "big" dependencies : I know it has been done with xtext (which means removing the DSL, and probably move the configuration and the rules in UI, java code or the new rule engine) but there are probably other libraries that could be removed : guava, guice, jersey, ... (these 3 only amounts to one third of the size of the minimal OH2 runtime)
  
• run on a smaller OSGI Framework : I've read that concierge was a potential target, but is that already a reality ? I don't see a lot of activity on the project and I could not find any information on it's current status. Knopflerfish could also be an option, has anybody already tried it ?
  
• run on a smaller JVM : either javaSE Embedded 8 (any luck on that ? ) or any of the other embedded jvm ?
  

• removing all "big" dependencies :
  Guava, Guice: these are dependencies which are essential part of the runtime and will be used by some bundles internal. It will simplify development and avoid redundancy using same core functionality. There might be optimizations possible to downsize these libraries to required parts only.
  Jersey: Jersey is required as any UI connecting to ESH does need REST APIs. In theory Jersey could be replaced by another JAX-RS container if required. We did some investigation for more lightweight REST frameworks, but there was no solution easily replacing Jersey (like Apache Wink, does support JAX-RX 1.x only, no SSE).
  FYI: there are PullRequests to OSGi-JAX-RS-Connector to downsize and optimize Jersey for using in embedded environments. See project OSGi-JAX-RS-Connector from Holger Staudacher, pull requests 91, 93
  
  It may depend on your requirements whether you need REST APIs at all: then you can get rid of some bundles related to Jersey and REST in general.
  
  
• run on a smaller OSGI Framework
  I am doing the port of Eclipse SmartHome to Concierge. If you are interested in progress read the file "JochenHiller/concierge-tests/blob/master/docs/Concierge-EclipseSmartHome.md" at GitHub about the current state. I am running minimal OH2 runtime with Concierge. But to be honest: the OSGi framework is the smallest part regarding the footprint.
  I am actually waiting for release builds of Concierge, Eclipse Mars and Jetty 9.3 to provide a PR to Eclipse SmartHome for supporting Concierge, as most problems have been in conjunction of Concierge and 3rd party libraries.
  
  I personally did not try Knopflerfish. I heard that some developers are using ESH with Apache Felix/Karaf.
  
  
• run on a smaller JVM :
  We are running ESH on JavaSE Embedded 7. Future plans should target JavaSE Embedded 8 too, we think that Compact profile 2 should be sufficient. I did not made a full testing against full JavaSE Embedded 8, but expecting no problems. For compact profile 2 there might be small changes which I do not oversee until now.
  I did some testing with other JVMs too, which did work for the core framework. But to be honest: there are not so much embedded JVMs available on the market which supports Java 7 language/libraries and later.