Would it be a reasonable trade off for a Synthesist Summoner to be somewhere between two and four levels behind on your Summon Monster SLA in return for being able to mostly (but not 100%) hide their melded eidolon then un-hide it for a full round action or two?
If not, then is there anything else that would be an appropriate trade-off for such an ability?
By being behind on the Summon SLA, I mean getting no ability to use it until level 3-5, at which point you get Summon Monster I, then continuing on with it as if you were however many levels behind, getting the new bit every two levels.
By not 100% hidden, I mean small elements peeking through that the observant can notice, and the knowledgeable can realise the implications of. Probably spot to notice, then knowledge arcana to work out what it means. Possibly either.
The one to two full round actions would look something this, and I'd say they could be disrupted by hitting you, it'd be a concentration check to not fail the thing, and have to do it again.
The reason for this being I'm going to be in a game with a fair bit of urban stuff where my melded form would not be appreciated, so the DM was thinking of some kind of quick change ability. Yes, I could just use my SLA summons instead, but I'm planning on mixing Barbarian levels into it, thus my SLAs will drop behind quickly.