Having 4 systems to chose from is hard,… so lets add another to make things even more confusing:upside_down_face: .
I want to share a very loosely designed transaction “Buddy System” 
. The idea is that you can have a buddy who handles the transaction for you, functioning as a Relay/Intermediary. This relay requires your signature to finish the transaction and get his reward, which is a modest relay fee. The proposed solution has some advantages, such as being an Aggregation of a regular RSR and SRS transactions, not requiring side chains, merged mining or much code. The system also has some downsides, such as the Relay knowing who sends to who, although an extra tor address could provide extra anonymity there. Also this idea requires some transaction information wrapper around a slatepack address to define this alternative transaction method.
To be very clear:
1) I am not a cryptographer
2) I am not advocating this solution should ever see the light of day
I designed/explored this solution for fun since I believe there is a wide range of use cases and possibilities using interactive crypto that are worth it to be further explored. Grin is amazing in that transactions can be aggregated at so many levels, this is just one example of an application where aggregation can be interesting.,
Grin is working as it is and I am not convinced any formal transaction relay system or Bulletin Board System is needed. Perhaps integration with Nostr or Signal on the individual wallet level would be more important as proposed by @trab. This does not however mean we should not continue exploring solutions like the Buddy System presented here. I think there are uses cases for using cut-through for aggregating transaction before they hit the chain. Sharing ideas like this one might helps keep everyone open-minded and creative and lead to new cool crypto solutions one day… that is why I share this concept idea.
If you are interested, give the transaction “Buddy System” a read 
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Grin for muggles and aspiring wizards [github]